The JCM 900 circuit really has a distortion box built in. It uses op amps to do most of the work, two diode clipping circuits one LED and one using rectifier diodes. Then it runs the signal through a couple of tube stages before the effects loop. Reverb is also driven via solid state op amps.
I'm not entirely sure the approach taken by Marshall in this amp is a bad way to go really and it was certainly less expensive to build. I mean if I put my TS808 Tube Screamer in front of my Marshall 50 watt non master volume head it sounds good with the pedal giving the amp a little extra "kick". This is just built into the JCM900 and you can't bypass or remove it from the circuit. It will NOT give you the non master volume Marshall sound but I don't think it intends to anyway. It's also kind of cool how it uses what looks like a preset tone stack in the lead channel that changes the frequency emphasis on that channel while still retaining the tone controls that are shared by each channel.
In an amp like this I really think you could choose the output tube based almost entirely on reliability and longevity since in the case of most users it will contribute little to the tone. I'd say a KT88, 6550, 6L6 or EL34 could all be used without trouble or a whole heck of a lot of difference in the sound. What is interesting is that with the clean channel master volume all the way up the amp seems to have nice dynamics and the tone is not bad.
A lot of power amp distortion is in the phase inverter section, yes. But with that sort of master volume you still remove the output transformer from the picture and it does play a role at least when it comes to sculpting the frequency response of the amp. It'll still do this but since it's never driven to saturation it won't sound quite the same.
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