Hey Luke, thanks for the props. The good news is (very good news at that), is that you are in the right place to learn the Plexi circuit, Marshall Amps, etc. Collectively, I think the vast majority here would agree that a stock 68 Plexi circuit is the foundation of it all. I happen to start my journey with a JTM-45 Re-issue as Phil (Strat78) said but it's all been changed to a 68 circuit with only two tubes and an oddball, but effective OPT Transformer. From there it's variac's, voltages, Guitar/PU and speakers. An accurate EP-3 pre-amp circuit is also used in my setup but you'll hear many here killing it without.
As it stands now, my setup is the same. I'm always dinkin around with old PU's that I might find in pawn shops or parts bins from music shops but my favorite is still the setup from the clip so far. I don't do the re-amp thing anymore because this is superior IMO. If it's really late at night and I feel like playing quietly, I just practice through an old little Samick amp that a friend found in a dumpster like 5 years ago. With an old Boss PQ-4, pedal, you can make any amp sound pretty wicked.
Phil, I think dialing a variac to 90VAC, does do something to the tone but really only if the amp is left biased at 110V or 120V. Are you hearing it the same when dropping to 90V? This is interesting to me.
Oh, I aways forget. This amp does still use a GZ-34 tube rectifier, I can swap that in about 5 seconds flat but I just like the way it sounds and feels so I leave it. The OPT Trans and GZ-34 are the only 2 parts in the JTM-45 DNA. All else is 68 era circuit.