silentrage said:
Even if you do get the MIDI switching, you're still stuck with the shared gain and EQ (I mean the volume/bass/treble/mids, not the GEQ) and the compromises this forces you to make, especially when trying to get three volume-balanced channels.
I think the selling point of the Mark III isn't necessarily that it's the greatest amp available, it's that it's a ridiculously good-sounding amp for what you pay for it. There is no, repeat no, better amp available for under $1200 or so. You can dial in any one channel to sound as good as any $2500 amp. However, that $2500 amp will have at least one other really good-sounding channel, one or more switchable fx loops, some kind of onboard boost, etc.
If you don't already own a MIDI switching system, you're kind of getting into the question of whether you want to drop another $800 to get modern switching for a 20-year-old amp that cost you $750, or just flip the III and use the cash you were going to spend on the MIDI to buy, say, a used Roadster and some EL34s, which will give you a much more versatile amp overall.
I love my Mark IIIs but they're 20-year-old amps based on 25 or 30-year-old designs. You can only do so much within that.