I don't have direct experience with the M901, just jumping in as this tends to be a quiet board. I have a ONE.3, and it's a little workhorse for a daily dose of nightly music or Netflix.
I think the price and performance of the M901 made it a great entry-level amp, or a great step up from entry level, when it was released. Its relative age simply means that a lot more amps have come on the market since and many owners are probably feeling the itch to step up or over into something different. Some may be liquidating systems for various reasons. I seriously doubt seeing M901s for sale means that the owners are dissatisfied.
Hopefully some M901 owners will chime in. It's just my theory that owners of Virtue amps tend to do more listening than surfing the boards.