Have you started sifting through all the information here http://ampgarage.com/forum/viewforum.php?f=4&sid=6cb83cde085b2d8ac60c5815564d2cd7

What speakers are you using? I have an HRM50 and so far haven't had an issue with the bottom end. Although I haven't played it really loud yet. I have played it loud enough to hold up with a drummer, but not pushing it to it's limit. But I do know someone out there has modded theirs to address every problem they thought they had.

That's a good speaker for the OTS, but it is a darker sounding speaker than others. Are you still having issues with "mushy bottm" on your OTS?

One other thing, it's probably not best to do any mods until you have a good 20 or 30 hrs. of playing time. These amps need a good break-in period.

Agree with ya'll

Break in period is very important with these amps - and also when/if you change tubes, it takes a little play time to get it broken in.

The Celestion 12G65 are pretty hard to beat with these amps.

I have a few other cabinets with greenbacks, EV, Fane, ToneTubby, but my grab and go cabinet is a 1x12 with a 12G65 that I bought on ebay a couple of years ago. I asume the weber you mention as well as the WSG sound the same as the Cel 12G65.

Determining any sonic improvement to an amp without hearing in person is so difficult - interpretation / playing style / personal taste etc.

In my view the stock OTS like it's Dumble cousin has a very 'full' bass that is more prone to mushiness than other designs especially in OD mode.

Things to consider.

1. Is there too much bass without OD engaged?

2. Can you 'dial out' the bass with the bass control

3. Are you using open or closed back cabinet

4. Is cabinet placed close to a wall / corner?

At least you have played the amp at gig levels - I thought my amp was bass heavy until I played a gig, then I found it to be just fine.

5.You can adjust the bass content in various ways but you need to remember that some changes act on just the OD sound whilst others are 'global' and will change the clean sound as well.

An easy one to implement is to change the 10k resistor that links the Bass control to ground. This effectively sets the minimum level of bass (when the control is set at zero). If you change this to 1K, you should find that you have a bit more control over the bass content at the lower end of the bass control range.

If that doesn't help then you need to think about coupling capacitors. These link the various stages on the amp together. Between each valve (tube) stage there is a coupling cap, if you reduce the value of these, you will lower the bass content. Changing the value between V1a and V1b or between V1b and V2 will reduce bass 'globally' I.e your clean and OD character will change. Lowering the capacitor value between V2a and V2b or V2b and V3 will impact on OD only. I'm afraid that component substitution by trial and error is the only way. If the existing capacitor has a value of .05 then try .02 (basically shoot for half the current value - depending on the original capacitor value, you may find that there is one half way between.

Finally, some have found that changing the power supply filter caps for higher value or 'high quality' brand such as F&T helps to tighten up the bass when the amp is played loud.

Unless your speaker is in a closed back cab, I'd be surprised if changing it will dramatically change the bass character.

Has anyone tried using a 12AT7 as the phase inverter? It is suggested that these will clean up the bottom end in Fender amps....and no modifications required.

If i recall correctly, weren't you running KT-88's for a while? Did they help with the low end at higher volume levels?  Also, did you or have you heard of anyone trying KT-77's?