6L6's IMO sound loose no matter what guitar amp theyre in, try some EL34's or KT77's to tighten up the bass

depending on what you want to do with it. you can either plug into the normal input and use the mesa preamp like a pedal (activating the PAB will bypass the tonestack to let the mesa shine through more) or plug the mesa into the effects loop input which will bypass the preamp and just use the poweramp

here a mod sheet i did in paint which is why it looks corny

the v2B is one of the caps you mentioned. The V2b is the one closer to the powertubes and PI. To parrallel the cap just wire a second cap exactly like the 270pF (but leave the 270 there) you now have two caps in parrallel which will increase the total capatiance. (add the value of the 2 caps for the total value, you want something close to 600pF but the exact value is not too critical)

parrallel a 300pf(or similar value) ceramic cap across with the v2b snubber. This will take some high end of the OD. You could raise the value of both snubbers but i prefer raising the v2b because IMO it gives a more dumbly tone. I also changed the choke to a fender blackface twin choke. It made a very small difference and it's only noticable at higher volumes, but it seems to tighten up the bass slightly, 6L6's sound very loose to me

S&M is much brighter on the OD and sounds more agressive. The normal OTS was modded to sound thicker and warmer, it's also supposed to be better for switching "on the go" meaning that it's much more balanced between the OD and clean. For example, the S&M might be to muddy on the clean channel so you increase the treble, this will also make the OD channel brighter and it could become harsh.

I have an OTS and even though it has the mods to warm the OD channel up i had to add a few mods to thicken it up a bit more.

I attribute that low end looseness to the 6L6's try either EL34's, KT77's or my personal favorite KT88's

Like i said i've used them without rebaising and they run fine. Since im pushing these very hard i lost a small amount of clean headroom but the sound was tighter and added a bit of smooth compression. At higher volumes they added a small amount of "grain" which you may or may not like. For low volume playing i like the 6L6's better but after a certain point they sound too loose and the bass is too flabby for me.

The OL Model (early 80's):
Biacrown's "high gain" model moved back to the Input 1 circuit (still keeping PI 2), but used the "extra" half triode as an additional gain stage.

correct me if im wrong, but i think you could just use the filament heater 6.3V tap for the relays. My non ceriatone Dumble build does this and works fine

i have an 80's all original hiwatt Lead 50 that i'd be interested in trading. Now before you say anything i know you want a JCM800/Marshall type amp which is why i offered this. The 80's hiwatt i have has a very similar circuit to a JCM800 and sounds very much like one with the gain up, the cleans are still very much hiwatt like. This amp takes pedals amazingly and has an original effects loop and is hand wired point to point. both things that original JCM800's did not have.

let me know if youre interested we can work something out

IMO the amp just needs to be broken in and could probably use some warmer sounding speakers. I had a similar problem with my non-HRM OTS, the HRM should sound warmer due to it being an HRM and having the internal treble control. Just give it time, the tubes as well as all the components need to break in before they sound their best. after you put about 20 hours on the amp if the amp is still too bright you might need to try some g12-65 or other warmer sounding speakers. Those old 70's speakers can be a bit bright with certain amps. It might sound warmer with a fender due to the mid scoop but the OTS has much more mids which can be percieved as very bright with some speakers

Thats why i suggested to lower the overdrive trim pot. Also Let the amp break in dude. it will lose that fizz and tighten up the bass. What speakers ar you using with the amp?

the OD channel shouldnt be that bright. especially on an HRM.

What speakers are you using?
Where do you have the OD trim at? (Gain trimmer)

Speakers are a big part of this amps sound if youre using vintage 30's or something expect a bright sound.
Also the OD trim pot being too high will result in a harsher less dumble tone.

Also i assume you just got this amp? Try letting it (burn in) is will become warmer, smoother and more "touchy"

im using a hammond choke and OT with a weber PT for my KT-88 build