I recently acquired an MC250, had my '71 twin modded for preamp-out, and played 3 shows - 2 shows on Friday 6/7: one at the Portland Rose Festival on a fairly large stage with a solid sound system and great monitors (with Pete B on steel!), Again on Friday evening at a local bar (small stage), and lastly on Saturday eve at another local bar - a total of 5 sets of GD. All 3 of these shows were the first "test drives" of my new twin/mc250 combo.
My immediate impressions are that it is a lot easier to get a solid-feeling pick attack than previously. In other words I have to do less work / use less effort to create a solid volume via my picking. Also there is a distinct "tightening" of the lower notes (5th and 6th strings below the 7th fret) - both feeling and sound. The notes created in this register 'pop' more, have more body and character, and have a subtle grit to them. Also there is now an overall feeling of slight compression that didn't exist before, like everything is being squished slightly - not in a bad way! ...it's more of a feeling than something audible; it goes with what I was saying before about having to do less work... It's cool and kinda feels easier to get the right "feel"out of the guitar - very fun to play!
The overall sound is really quite an improvement as well. The highs, mids, and lows all seem to have become 'juicier', and more 'present' - It took a few tunes to become accustomed to it... The highs especially have a crystal-like "jingle" to them that is a dramatic change from previously, and is one of the 'core' components of the Jerry sound I had been missing. Also the mids/low-mids have a certain tonal quality that is more 'tight' or 'pressurized', with that weird subtle compression... Again, 'very' Jerry. My effects reacted as expected. Everything fell in line with little adjustment (luckily!), and I find I can get that unmistakable 'Jerry clipping' simply by digging in hard and having the MC up loud enough, which fortunately, is not 'too' loud. I find that running the MC volume a bit hot, and playing very dynamically/lightly is the trick to much of the subtlety of the Mac.
All in all, adding the MC250 to my rig was a dramatic improvement in nearly every way. It was a seamless transition with little-to-no tweaking needed; everything fell into place effortlessly. The only difficulty worth mentioning (aside from carrying the thing) is with the reverb. I struggled a bit getting it just right - it has a different dynamic to it that i was not expecting - cant really explain it other than to say it 'feels' different...
I can honestly say that I was skeptical as to whether MCs were truly "all that"... Now, I have no doubt - at least in my limited experience with my MC250 and power amps in general - the MC is an absolutely critical piece of the Jerry puzzle.
(Ps - ill post some before/after audio/video clips soon)