I asked Cliff if the 'Modern' mode on the 2:90 was the 'Voicing' switch he was talking about but didn't get a response. I am assuming that is what he is referring to since the Deep mode just adds extra bass and the 1/2 drive is self explanatory.
Edit: I just checked the manual for the 2:90 and it looks like the 'Modern' mode is indeed what he is referring to. From the manual:
MODERN
Again, this LED indicates the Modern voicing feature has been selected.
Derived from the presence shift switches found on our Mark IV amplifier, and the "Red" or Modern Channel of our Dual Rectifier Solo Head, this sound pushes at the boundaries of today's high gain crunch. To the tame and timid, Modern is a scary sound. It is the opposite of soft... it's hard... hard core aggression. If you love to get way agro with super gained out crunch, Modern will re-focus your attack and clean up the slop. Often times when trying to dial the amount of gain necessary to get the scream and grind... you quickly find the low notes falling apart or becoming too washy. Modern is the cure for "anemic chunk”. It places emphasis on the percussive mids and highs and tightens those flubby lows where they tighten up right at the power section. Modern also provides a little more headroom resulting in a volume increase. When Modern is used in conjunction with Half Drive there is virtually no volume change and the benefits of both are incorporated to achieve more gain, more aggression, more harmonics, more scoop, more attack etc. Then try the ultimate enhancement Deep, Half Drive and Modern! We'll spare the adjectives ... just try it out and you'll see why we think this is the most advanced and easiest to use two spaces of Tone Power on the planet.
http://www.mesaboogie.com/media/User Manuals/Stereo Simul 2-90.pdf