I have the M51 in my system and while I don't use your amp or speakers, here is a review that has the Rotel RB-1582 MKII and A23 in direct comparison.
http://www.soundstageaccess.com/index.php/equipment-reviews/560-parasound-halo-a-23-stereo-amplifier
They consider the Rotel a bit warmish, softening leading edges and not having as much resolution or imaging capabilities. The Parasound was preferred, but it likely wouldn't help with upper treble remarks you had voiced.
I've never owned either brand, so I couldn't give a proper recommendation on them. As for the M51, bags of resolution while never sound harsh and even a bit of warmth. Used it direct and with my preamp and it seems very close either way. I keep the preamp in for added input flexibility.
http://www.soundstageaccess.com/index.php/equipment-reviews/560-parasound-halo-a-23-stereo-amplifier
They consider the Rotel a bit warmish, softening leading edges and not having as much resolution or imaging capabilities. The Parasound was preferred, but it likely wouldn't help with upper treble remarks you had voiced.
I've never owned either brand, so I couldn't give a proper recommendation on them. As for the M51, bags of resolution while never sound harsh and even a bit of warmth. Used it direct and with my preamp and it seems very close either way. I keep the preamp in for added input flexibility.