I have two of the m900u. The difference between running one amp in stereo vs two in mono is striking in what it does to the sound stage. I don't play it that loud, so don't need the power, but felt the soundstage/air was worth getting two.
I compared a single to a Gamut 200 (nice amp that I had had for 6 years).
I have VERY good bass on my system (Focal Scala) and they are not strident or sibilant (clean but sweet highs).
YMMV in your system.