I can't say enough good things about GabKits. 

I have their OTS 1x12 Combo Cabinet loaded with a Warehouse Guitar Speakers ET65 for my grab and go rig. 

It sounds excellent with my Strats for SRV tones and for RF sounds with a Les Paul. 

This speaker has a full, smooth combination of highs, mids and lows.  Excellent all around tone with power handling to keep up with 50 watts.

I have tried a number of speakers with my OTS and have settled on this for my go to 1x12 set up.

Here's why:

EVM 12L - Expensive, heavy.  I see why some people love these, but it is definitely not for everyone.  Very "in your face" tone which is somewhat typical of high wattage speakers.

JBL D120 - Hard to find.  Pricey.  Nice.  A bit too much headroom and not enough compression for my taste.

Celestion Vintage 30 - Great all around speaker.  One of my favorites.  Plenty of highs if you want them.  Perhaps a tad light on the lows.

Hellatone 60 - "Broken in" Celestion vintage 30 from Avatar.  Great Bargain.  Another of my faves.  pretty much the same as above, just a little easier to find the same tone at a slightly cheaper price.

Warehouse (WGS) Veteran 30 - Another great deal for the Vintage 30 tone at a reasonable price.

WGS Retro 30 - 75 watt version of the Veteran 30.  Much stiffer sound.  Not for me.

Weber Alnico Blue - In this same vintage 30 ballpark as above.  Excellent top end. Very nice but a bit pricey.

Celestion G12M "Greenback" - Also nice but arguably underpowered for a 50 watt combo.  A little less high end than the above speakers but smooth with nice breakup.  The sound clips of the WGS version of this speaker also sound real good.

WGS ET65 -  You see the Celestion G1265 mentioned a lot on these forums.  I decided to try one of these just to see how it sounded and was surprised that it became my number one choice for using just one 12 inch speaker.  It sort of has the character of the Celestion G12H30 and Vintage 30 combination that I use in my Koch Multitone 2x12.  This combo has become very popular with a number of builders for mixed 12s in a 2x12 cabinet, and with good reason.

I had expected to like one of the Vintage 30 type speakers the best, but when It came down to picking just one speaker for the combo cabinet, this turned out to be the one that delivered the goods for me.

Could a B/F style amp incorporate the older 'variable bias' style tremolo?  No cockroach necessary.

I've heard good things about something called a Surf Bug opto isolator, too for that style of trem. 

The PI section from the Princeton Reverb can add a nice type of distortion in that part of the circuit for those looking for the lower volume grind.

A hybrid B/F for recording and small to medium stages?

As much as I love my old amps, they are not getting used much since I got my OTS.

Thanks for all the great amps, Nik.

OK,  I changed both PAB resistors to 4.7M.  Thanks to mcinku for the help.  I would have missed the fact that there are two resistors to change.

It's tighter than it looks getting a soldering iron in there behind the switches so think twice about doing this and mask things off so you don't burn adjacent wiring.  I disconnected the Orange Drop from the switch, moved it out of the way temporarily to put in the new parts and then re-connected it once the resistors were in.

My first impression is that with the 4.7M resistors I get less of a volume boost from the PAB and more of a mid boost than with the original 22M resistors.  However, your mileage will vary depending on how you set up your amp.  All the controls in and on the amp play a part in making this work.

The jump in volume from disengaging the Tone Stack with the PreAmp Boost depends on how you set your Treble, Middle and Bass controls.  What I mean is, if the levels on the ToneStack controls are set low and then you disengage the ToneStack by clicking on the PreAmp Boost, you get a big jump in volume.  If you have the controls up a bit there is less of a volume boost when the tone stack is bypassed.

My goal was to get all four functions (Clean, Clean + PAB, Overdrive, Overdrive + PAB) to be relatively close in volume with the Boost being a little louder but still similar in tone to the regular channel with the ToneStack engaged.  It's a balancing act that includes all the controls and switches, but I feel like with the 4.7M resistors I'm closer to having four useable functions than I was before.

I listened to each channel with the PAB on and used the Bright, Deep and Rock/Jazz switches along with Volume, Level, Ratio and Presence (basically everything but the tone controls) to get a PAB sound that worked with both channels.  Keep in mind that the Volume and Level controls are key to setting the both overdrive characteristics and the relative volume of the PAB.

Next I tried to get close to that basic tone in each channel using the ToneStack controls.  I ended up with the Volume about 6, Bright on, Deep off, Rock on, Treble 8, Mids10, Bass 8, Level 4.5, Ratio set so Clean and OD channels are consistent in volume, Master fairly low, Presence 8 to 10.  I play blues in the low to medium gain range and I like some chime on the top end.

I haven't played very loud with this yet and I'm not positive that the character of the overdrive has not been affected by the change of resistors.  I'll have to see as I play it more and play it louder.

But I have played both my number 1 Strat (Rocketfire pups) and my Les Paul (WCR American Steel Set) without changing settings on the amp and have gotten good tone from each guitar.  I mean, these amps sound pretty darn good no matter what. 

This amp had all the Zucker Mods when I got it but I put back the small bright cap on the Master Vol.  My internal Gain Trimmer is set at around 9:30. 

Right now my favorite all around speaker set up is a Hellatone 30 (G12H-30) together in the same cab with a Hellatone 60 (Vintage 30).  That's all I've had a chance to play through so far.

Hope this info is useful and thanks for the help.
Rich

Thanks.  Correct me if I'm wrong here before I solder anything.  One 22M resistor is on the small board behind the switches and the other is behind the Treble pot.  Both connected to the switches.  There is already a 4.7M resistor connected to the ground buss.  Leave that one as is.

Thanks for taking the time to weigh in on this.

I'm thinking in terms of something that would not affect the sound of either channel with the tone stack engaged but would allow you to dial in the tone and amount of boost that happens when the PAB comes on.  Seems like the first step would be to be able to decrease the amount of boost somewhat.

Have any of you circuitry tech gurus experimented with some kind of controls over the sound of the PAB?  Maybe along the lines of the Tw@ R@ck Lead Gain and Lead Master controls?  Maybe changing components on the PAB board so that the change is not so drastic when the PAB is Engaged.

I've done a lot of adjusting with my Zucker version amp and it's pretty well dialed in for my Strats (Lollar Blackface p/ups) and my Les Paul (WCR American Steel set). 

See if any of this is helpful.

I have the stock Tung Sol RI 6l6GC tubes which sound fine to me.  For years I have biased my Fender amps at around 32 to 35ma with good results.  The Weber Bias Charts and others recommend as high as 46ma or so for an amp with 450v plate voltage which is what my OTS reads.  I would suggest that you bias to somewhere between 35ma to about 40ma rather than to the hottest setting.  Mine sounds good to me all through this middle range.  For now I'm leaving it set at 40ma.

I have used Weber Bias Rites for years and I like them because they are easy to use and they show both the plate voltage and the bias.

With the Bias adjusted, set the Volume (far left) to about 7.5.  Right between 7 and 8.  Set the Presence to 8.5 or a little less.  Up to this point the Presence control adds harmonic distortion by decreasing negative feedback.  At this point and above it begins to add treble and very subtle adjustments in this range can help get the kind of edge you want on your OD.

Get as close as you can to what you are looking for by adjusting the Presence and then leave these settings alone while you adjust further with the OD trimmer inside the amp. 

I marked where mine was set when I got it and then turned it all the way down (toward the back of the amp).  Then I set the OD Level control on the front of the amp to about 2 (pointing straight over to the left) and the OD Ratio all the way up.  You will have no volume in the OD channel while the trimmer is down like this.   The idea is to turn the internal trimmer back up until your OD volume matches the Clean volume when you switch back and forth.  I end up adjusting it as low as possible but up just enough so you can hear some drive in the OD channel and the OD volume at this low setting still gets at least as loud as the Clean channel.  Mine ended up at around 9 to 930 which is a bit lower than where it was when I got it.

Now adjust so you have the sound you want on your Clean channel and the drive you want on your OD channel.

I get a nice cleanish overdrive with Strats at low drive settings all the way to heavy drive with a Les Paul and the OD Level all the way up (backing the ratio down accordingly).  I usually end up in the low to mid drive area personally but this set up gives a wide range of useable settings.  You can easily shift the range toward higher gain and hotter bias if that's your preference. 

Experimenting like this will also give you an idea about how the different controls work.

Probably nobody's interested at this point, but if you hook up your effects loop backwards your clean channel will disappear until you get the send and return right again.  The OD will still be there but a bit quieter and thinner than if it's hooked up correctly.

For what it's worth, I have found the Presence control to be a big key in achieving a subtle smooth drive with both single coils and humbuckers.  Since it controls the negative feedback circuit it can impart some drive and harmonic distortion to the clean channel. 

In the clean channel with the PAB off, adjust the tonestack to your preference.  Now use the Volume (far left) and the Presence control to get some added harmonic content and even some bite to the cleans.  Turning the Presence control clockwise decreases the amount of negative feedback added back in to the circuit and in so doing increases harmonic distortion.  Some of my favorite clean tones from Danny Gatton and Roy Buchanan to Stevie and Jimi aren't really all that clean if you listen carefully.

Now go to the Overdrive channel and adjust.  Switching between the two channels set up like this has helped me achieve some great tones that had always eluded me with other amps.  Some nice singing sustain to the cleans and some great greasy blues and rock tones.

Also, don't overlook the usefulness of the Jazz switch even if you play Blues or Rock.  This function will change your settings a bit to get your main sound (compared to the Rock function) but you may find it helps even things out when you bypass the tonestack with the PreAmp Boost.