you have asked the right person! hahahah my guitar teacher loves the m9 sounds and has a particular tone combination in which he mixes the mxr dynacomp with the tube drive. it gives you this kinda fluid high sustain low gain tone which resembles eric johnson's tone by quite a bit HOWEVER.
i find to really enjoy the m9s drives you still need to have a very responsive tube amp. though what im hearing is great from my ac4tv, the feel cannot match just running into the seventh alone. things like pick attack and all, i dont really know how to describe this but it just doesnt feel very right. it feels like whatever im playing is coming out the same, and a bit of expression is lost. it feels unatural when im digging into the strings.
i've tried everything i could to get a more amp-like response, and it improves when i play live in church with a huger amp, but in no way as dynamic as the seventh. i've tried convincing myself i dont need the seventh and i can live with the m9 only, but i think i wont live a very satisfied person! Noise is also a teeny weeny issue with the drives..
my conclusion is that you need to test drive the m9 for a few days at least to be able to feel the difference.. yep! and also it wont feel very awesome through small bedroom amps.