How much difference do you think it makes, is it worth upgrading to the Mercury one?
They're both Marshall 2668 clones, so it won't make that big of a difference. It will just come down to the type of steel and internals. I'm thinking back 20 years, but I believe the OT for the Classic was different. I could be wrong. I do know the 101b is a 2668 clone. I had a Classic and about 5 or 6 different 101b heads.
There was a guy in Europe that had a 101A, and he tracked down an older 2668 OT, put that in, changed what needed to be changed for the EL34s to work properly and off to the races he went. That would be the cheapest route. If you do go the Mercury route, it would be wise to get one where the mounts are the side and spaced right.
I like them all, but I tended to like the pre-2004 Red Channel the most. It sounded like a bigger Blue channel to me. The Cameron mod adds compression. The Post-2004 mod adds compression but has a different voicing. glpg80, brentSP, 46&2 and others around here can look at a schematic and tell you the differences based off of that. After 25 years of tinkering around, I'm just now deep diving off into that area.
Anyway, both of those mods are a resistor over a cap or a cap over a resistor at one spot in the amp (remember 20 years). If you snip off whichever it is on top, you're back to pre-2004. The Marvel mod lets you swap among all 3. Brent had one where he called it the Uber-mod, but, last I saw, he was using pre-2004. He was one of the main people that talked me into getting the 101b back around 2002 or 2003.
I love the old school type of power section of the 101B, and I love the Simul-Class power sections from Mesa. If you have good preamps you like (IE: Marshall JMP-1, AxeFx, etc.), they will sound great running through those power sections. I did that for a little bit.
Backtracking to the Classic vs. 101b... The Classic is a big more open, less compressed and a little rougher around the edges. Bob Savage used to describe it as "toothy". I agree with what he said. Other than those characteristics, you can tell both are Bogner Ecstasy heads. The Classic has the high headroom cleans, the 101b's clean preamp is a Super/Deluxe clean which Jccobb2 mentioned earlier.
That's some rocket fuel for some of you. Some of these more knowledgeable guys can chime in with some of the more technical corrections and details.
The 101b is a great amp for what it does.