Have you tried messing around with the presence knob?  That works on the OD channel with mine, a more fine adjustment than switches.  I have also tried PVB's switch settings myself while running through a 2/12 hemp cone cab.

You are welcome. 

And your carefully explained thoughts are illuminating. you have explained your self in ways so that others can help.   

My 50w HRM only has the LNFB mod, has a C-lator hooked up sitting on top.   That amp/lator and my choice of vintage old stock tubes and various speaker cabs...sings at small room volumes quite easily.   I can get a lot of bounce and still get the singing, note flipping, oozy lower gain stuff or adjust trim and gain on the OD channel, get some really good old school Marshall-y, more compressed tones.   

S'allgood.

Ummm, actually, the ability to run a D-style amp in completely unmodified form gives up (from the right set of fingers) a truly staggering palette in both variety and real quality of tones.  If you put in every mod you read about you just might ending up compromising a design that was designed by one much more talented and experienced in the art of modding an amp...HAD hisself. 

Regarding your inputs,  a/b/y switch?  I use one and love what it does for little $$

Now that is a post of posts!  Yaaaayyuuuhh!!  Very ampgarage-ishly accurate.

Mount it up, play through it, and let us know!

Man, you just started on a big-ass journey...hold onto yer butt!   There will be many new things happening to your playing.  Don't be afraid to test old and new ideas, then discard  "nonworkingforyou" and add "onesthatdo" as you explore.  One of the bennies from getting to know my 50w HRM was learning to set up the controls on a couple of other, very nice, amps to get some of that D-tone singing thang going on, for instance. 

Serious congratulations to you!

Sometimes the speakers have a lot of change in their tone between low and high volumes, some ceramic magnet speakers can get pretty obnoxiously bright when cranked up.  Also it's the absolute nature of master volume amps for a given distortion/gain level to sound excessive at high volumes.  At high volumes turn the pre gain down some.  Just try it.

Just now I saw the latest post from the OP, so take my words with a big-assed shovel of salt...but once you get your PI "sitchiation" taken care of...

IMHO, +1 on the imbalance thang.  3-5mv not a biggie.

When I had my 50w HRM built I asked Nik to set the PI trimmer for a PI tube having matched triodes at time of build.  I never have touched it nor have I fiddled with the HRM tonestack.  Being an OCD-ish tube collector, I have a fave pair of 6bg6ga Sylvania grayplates (repinned 7581a's with a plate cap) in adapters, also 3mv apart.  I swapped those back and forth in the tube sockets and in my case my amp sounded best with the tube drawing less current closest to the PI.  You might try that and see if you can hear any difference, in my case I could as far as sustain/bloom was concerned.  One way or the other...

Congratulations on your build, my hat is off to you, and to all those who built their own D-clone(s)!

Great thread and great posts here!   The only thing I would add is not to get too terribly OCD-ish about nailing those tones your are seeking right away...get 100 hours on the amp first to break it in.  Not only do things like caps break in and form, as do new speakers and tubes, but the output transformer needs to season some as well.   So the tones will drift for a while until things settle in.

In 12 inch, some of my fave speakers are no longer made.  They are JBL G125A-8 and JBL MI-12.  G125 has 3" VC, rated 200w rms, heavy, MI-12 has 2"VC, rated for 150 watts and they don't weigh all that much either.    You can look them up in JBL's "obsolete" speaker brochures online.  They are wonderful for the tones you want.   Might be tough to find.  There is a custom speaker builder on ebay that does up some really nice hemp cone 12's too, and they are affordable.  Hemp speakers do that smooth OD and can do it at slightly higher gain settings than traditional speaker cone materials.  Harmonics ooze out nicely.  You might need to use increased treble/presence settings with hemp.  I also have a thang for using 15's...have some faves there, but not going to muddy up the water any more right now.

The best part is not in attaining JM's tones, IMHO, no, not at all...best is to get that personal "StratUltra" tone palette.  You certainly have a lot to play with!  Do some mad science experiments on your own, better than to just take everyone's word.  The results will satisfy your curiosity and you will know more! 

That's just plain purty...nice gutshots!

The classic speakers often used by D-clone users are the Celestion g12-65 and the EVM 12l.  I have found something better than those, but out of production...The MI and G series speakers from JBL.  Specifically, in the MI series, the MI-12 and the MI-15, in the G-series, the G125-8 12" and the G135-8 15".  MI series have a 2" flattened ribbon wire VC, rated for 150w RMS, the G-series use a 3" flattened ribbon wire VC rated for 200w rms.  All of these speakers sound great at lower volumes, and simply tremendous at higher volumes.  There is great control over the cone, no wierd sonic artifacts.  Clean or dirty, I have not found the equal to these speakers and I have heard LOTS of speakers.  Easiest to find on ebay.  There are some NOS MI-15's on the bay right now, I got two of them and they are wonderful. 

My 50w HRM has the LNFB mod. I like it.  That amp is wonderfully versatile.  Am running vintage 12ax7's and el34's, simply gorgeous singing!

I have two tube stores.  Main one is ebay, other is a private tube vendor who is a friend of mine an hour drive away. 

Started collecting my tube stash 6 years ago, got all I want.  Also have some nice vintage speakers, JBL, Altec, Jensen, CTS, Utah, Ampex/JBL in 8, 12, and 15's.  Have a few decent cabs in differing configuratins.  Benefit of all this crap is to know how tone is made and how to change it in the various ways possible to get a certain result with that given shop of tools.  So tubes are a part of the picture, not as important as the output transformer, that has more effect on tone, it's just a pain in the butt to change out every few minutes for another one to hear the differences...

In my Ceriatone/Marsh Overlord FSX-50 OTS  here is my list:

For V1, I wanted a tube with a tendency to generate lots of harmonic content, both odd as well as even and quiet for microphonics.  So I went Euro with a Canadian made Electrohome 12ax7a/7025a, the Canadian made and superior version of the 12ax7 shortplate Amperex "Bugle Boy" tube.

For V2, I wanted increased harmonic content, with no mud on bottom, yet fat and warm sounding at the same time.  A Raytheon JRP 12ax7 long blackplate with rectangular getter halo deliveres more of the fundamental note in it's tone and harmonic content, this will complement the Euro 12ax7 tendency for expressing more odd order content than the American tube.

For the PI, I chose a Siemens e83cc with closely matched sections.  This is the very top of the line for the Philips family of tube manufactureres, exceeding even the famed 10m Mullards for build quality as well as tone, I have both. 

Rich, full, sustain-y...

Power section:  6l6gc Sylvanias matched within 1/2mv @ 38mv bias. or 6bg6ga with adapters, biased at a similar amount, not as tightly biased, more like 3mv apart. 

This amp is very big, fat, and in your face with the Lift Negative Feedback switch,  goes from creamy to face ripping Marshall tones dependeing on which way that switch is facing and the OD trim setting.  Doesn't need a lot of OD trim or gain to be really agressive with LNFB.  This amp sounds better without the C-lator. 

For the C-tone 50w HRM/c-lator, I went totally  bonkers and got what I wanted, a Euro sounding el34 amp but having the smooth D-style oozyness of note flipping and singing overtone-ics. 

For V1, I chose an Adzam 12ax7s, a very rare Euro tube with very complex and abundant harmonic structure, with upper mids, top, and presence detailed like a silverplate mil spec Mazda 12ax7, (!!!) that is to say, abundant and sweet, and reasonably resistant to microphonics.  This tube, like all tubes in all sections of the preamps in ALL of my amps has to win a shootout among similar types to fill that spot.

For V2, a tube that will take what V1 hands it and turn it into monster sauce, I chose a Blackburn Microtech E813cc, a 12ax7 variant that was  a factory sample given to me byu the rep.  It is specifically designed to be placed in v2 and cathode follower stages in musical instrument preamps.  It is from what was the original Mullard factory in England.  The restart failed as a business, but the 10,000 hour rated tubes do their job well enough to win the shootout for that spot.

For the C-lator, now the V3 position, I chose a Siemens longplate 12ax7, this tube wants to sing and sustain like few others, in the class of the telefunken smooth and ribbed plate for delivering that wonderful overdriven violin sustain sound, along with a propensity to deliver the most amazing harmonics in volume swells.  These are microphonic as a rule, but this position is not as critical as V1.  No problems exist from microphonics.     

For the PI, I chose the Siemens e83cc in tightly matched sections for the same reasons as above.  Again, this tube won the shootout for its spot. 

Power tubes are a pair of tightly matched RFT/Siemens  el34's, matched within 3/4 of a mv, 37mv.  These tubes are warm, fat, and love to sing the british blues.

I am contemplating an OT swapout in this amp for a MM Radiospares enhanced model OT like in my Siegmund Midnight Blues for its' amazing harmonic content and clean tone gorgeousness.

...Ummm, you could do that but saving up for the real one is still worth it soundwise.  I played out in a cool dive bar with mine last night and it did great.

Yes, it does fit across the top with no overhang, almost like they were made for each other.  Perfect with a 50 watter, the Leslie G27 is rated for 100 watts RMS.