Hello James.

Interesting observation.  So I tried it!!!  LOL!!!

I THINK you could be right although I tested it a different way i.e. lowered the preset volume and raised the Master Volume (not the point you were trying to make I know).  I would say that there definitely is a difference, albeit slight, in the tone when the Master Volume is raised considerably.

I could not do what you're doing i.e. turning down guitar ouput doesn't drive the amp. (so your idea will work with clean sounds but you will lose your wonderful Marshall distortion for the most part).

One thing I found rather interesting:

Using the Marshall CODE PC Editor I lowered preset volume to 0.1 (you cannot do this with the knobs on the amp. i.e they jump from 0.0 to 0.2) (on my amps. anyway) and then increased the Master Volume to around almost maximum.  The interesting part: with the preset volume set so low the Master Volume becomes as linear as can be (so there goes my theory about bad pots. being used I guess).  Oh and a word of warning: if you ARE going to do this then please do make sure you don't forget that you've lowered the preset volume and turned the Master Volume up to these levels!!!  Switch to another preset at factory default settings with the Master Volume nearly on max. when it's the dead of night and you're just sitting around mucking about... well try it and you'll see why the warning (especially with a CODE50 or CODE100)!!!  LOL!!!

Regards,

Dale.

 P.S.  Forgot to mention though that turning the Master Volume up and either the guitar or preset volumes down results in way more noise from the amp. though.  I mic. up my amps. and when I tried this I did notice that all of my amps. (I have five) all generate quite a bit of audible noise when Master Volume is turned up very high (never bothered to see if it was just distortion presets or all though).  In a live environment you don't hear this at all but in a quiet recording environment it has the potential to be an issue (if you're not aware is all).