I’m just wondering ... why you would want to mod a Mesa to sound like a Marshall? It’s way easier to just buy a Marshall. They are **** near giving away DSL and TSL models on Reverb and Guitar Center.
If you want to keep the Mesa but get a more British tone, the first thing you would need to do is switch to EL34 power tubes (huge tonal difference from 6L6s). Make sure your amp has the ability to be re-biased for the EL34s (mine has a switch on the back and they strongly recommend only using Mesa Boogie branded tubes in matched sets).
For comparison... I have a JCM 800, a Bugera 6262, a Blackstar HT100, a TriRec, and a Nomad 100. I play them through Carvin(s), Bugera, and B52 4x12 cabs (I switch them around but mostly I play through the Carvin cabs because they have really great tone regardless of the amp ... and you can pick them up for next to nothing for a celestial loaded, all plywood, made in California, quality cabinet).
I love the Marshall when I’m playing classic rock and 80s metal... but it is a one trick pony and I still have to use OD pedals to get an 80s metal tone (like Ratt or Motley Crue). It has tube rectifiers in the power supply, which make it feel much looser and “vintage” than a modern Marshall amp with a diode rectifier.
Most of my serious metal head buddies all love the Bugera 6262 because of the tight tone and insane gain levels with everything set to 5. It goes from hot-rod crunch machine on channel 1 (early Van Halen/AC/DC type tones) to melt your face off insanity on channel 2 (most gain I’ve ever heard from an amp). It also has SEVEN 12AX7 preamp stages... so no mystery there. It has four EL34s and solid state rectifier, so it is really tight and punchy.
Personally I think the Blackstar has better sound than the Marshall for that tight British tone. It is kind of a hybrid tube amp. It has two 12AX7s and four EL34s, but most of the tone actually comes from digital circuits... not old school tube pre & power amp circuitry. It kind of reminds me of the Marshall AVT series amps, except it has a tube power section. It’s (in my opinion) a more compressed sound, and way easier to control your tone because everything in the digital circuitry is very linear. It is a great all around amp (like the Nomad) but less “breathy” and “snarly”... a bit flatter sounding (like if you used a compressor pedal in front of the preamp and another one in front of the power amp). This is my go to amp for noodling around at the house.
The rest of my metal head buddies love the TriRec on the 3rd channel with the mids scooped and the gain cranked. It basically sounds just like Dimebag Darrell. This is a great amp for metal tones, and channel 1 and 2 can clean up whenever you’re not melting faces. The Bugera 6262 still trounces it for highest gain, but the TriRec has that American Metal sound (courtesy of Mesa preamp design, and 6L6 power tubes) that has become increasingly popular over the last few decades. I also like being able to choose the type of rectifier... I use the pure tube on channels 1 and 2 (looser and less compressed feeling) and the solid state on channel 3 (tightens it up so the higher gain doesn’t sound muddy). By comparison the Bugera has less tonal variety and requires more control from the guitar (rolling off the volume) to change up the tone.
The the Nomad has my favorite tone because it feels grittier and more snarly. I got a Mesa for that very reason. It just feels raw and aggressive even at lower gain levels (it is basically a lower gain version of the which was purchased after the Nomad). This is my second go-to amp, but mostly because the tone is not as linear as the Blackstar. It has the same problem the Marshall has in that it sounds best in a very specific settings range (each channel has a sweet spot) and outside of that spot it doesn’t track linearly... so any adjustments have to be very fine and tested out before making any serious changes. I have each of the channels set in a specific way, and then try to make adjustments with my guitar instead of changing my amp settings.
As for getting metal tones from the Nomad: I do feel like I get more aggressive tones out of it when I’m playing a guitar with hot or active pickups. I also have a 10 band EQ on the front end; and a 10 band EQ, BBE, and an MXR compressor in my effects loop for shaping the tone to fit whatever I’m playing. I can leave the tone knobs at 5 across the board, use the 5 band built in EQ for a boost when I play leads (the solo boost only adds a touch of gain and just doesn’t feel like a big enough boost), the BBE for some sparkle and fuller bottom end on the clean channel, and the 2 EQ pedals for further tonal shaping. The compressor tightens everything up when I need to. I just use the EQs and compressor sparingly.
I suppose (in my opinion) you would be better off trying a few different amps rather than modding the circuitry on your Nomad. It was designed to fit a specific niche market and British tone metal really wasn’t the target audience. If you like Mesa, the rectifier series are good (I’d avoid the roadster 4 channel though), and the Mark V has an “extreme” voicing on it’s 3rd channel that sounds just like a rectifier series on the highest gain setting. I have seen the Mark V 90 watt heads for $800-$1200 on Reverb, and Guitar Center; and rectifier series heads for $500-$1500 depending on the model.
Good Luck!
