What kind of listening were you doing? Music or HT? To piggyback on Mark's comments, I have the M22s & EP175, and there was a noticable difference when I added my HSU sub. However (and I have nothing to back up this assertion), I do feel like there's just a greater presence that you can only get with floorstanding speakers.
And someone chime in here, please -- my understanding is that (with the exception of occasional peaks), most amps really aren't working all that hard under normal listening conditions.
