Is it possible to use some sort of AB box to switch between the the low and high input of a single channel JCM800, in a pseudo-channel switching manner? Are there any disadvantages to doing this (ie, a ridiculous volume difference)?
IIRC, when a cable is plugged into the low input of a JCM800 it is designed to automatically bypass the high input. I could be wrong though, hopefully one of the more techy users can definitively answer your question.
You can only plug into one input at a time.

If you plug into the high input it will bypass the low and vice versa.
I've read it is possible to moddify it so you can use both with an AB switch but I asked an amp tech and he said it wasn't feasible on mine.

You can get a great contrast by just using the volume/tone knobs on your guitar though.
You are all wrong. Plugging into the high input of a 2203/2204 turns the low input into a high input. They both connect to the same triode (unlike a Plexi) so what you get is two identical paralleled inputs to the same device. Well, almost. The high input has a 1M grid leak resistor and the low gets 1.068M grid leak. 10% is basically 0 in electronics so they may as well be identical for all practical purposes. There is no point using both inputs simultaneously on a 2203, no matter what you do.
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