Not good with Carvin S600, fantastic with everything else!
Today I went over to a friend's house to practice and since we were going to need three channels (his and my vocals and my guitar), I grabbed my Carvin S600 to use as a practice PA. My guitar with the M80 sounded just horrible through the S600. The S600 has this low mid resonant frequency that seems to emphasize a nearby resonant frequency in the M80. The Carvin's three band EQ is nowhere near precise enough to deal with this. I'm not exaggerating when I say this was nowhere near good enough to use even for practice. The S600 sounds fine with other guitar pickups. It resonates, but the resonance that it adds is usually missing from piezo equipped guitars and the net result is an improvement. With the M80, it's like both the guitar and the amp were adding the same resonance it was way too much.
Meanwhile the M80 sounded awesome with three different PA systems last week and it sounds absolutely glorious with my Henriksen Bud (which is my main amp for acoustics and archtops). The Henricksen Bud is really an amazing sounding amp. I may have to sell the S600.