new Parasound A21+ vs my old Rotel RB-1080

Don’t listen to someone’s semi-educated guess—especially if they have never owned or auditioned this particular model of Rotel.

Don’t listen to @lanx0003 who totally misses the point here. What he completely fails to realize is that the speakers just don’t possess the sound characteristics you’re looking for (as others have mentioned as well) so the amp is immaterial, but I think you’ve already realized that. I’ve listened to MLs with Bryston, Rotel, Arcam, and the result was always the same — they don’t have the bite/aggressiveness in portraying cymbals that you’re looking for and this is not a semi-educated guess but direct experience using high quality reference recordings. They just present cymbals in a different way — more diffuse and less focused. Not saying it’s better or worse as that’s a matter of personal taste, but an amp doesn’t change that.

just don't give enough of that... more than an educated guess.

Ok, I was trying to be nice but now I’ll just give it to you straight.  Your speakers won’t give you what you’re looking for because they can’t.  I worked for Magnolia for a bit and listened to a lotta ML speakers with my reference recordings, and they just don’t do what you want from them.  You can put any amp or any other piece of equipment in there and you’ll still be disappointed.  And that’s not a guess at all. 

I don’t think it’s the amp, I think it’s the speakers you’re hearing.  My friend has Martin Logans and the treble is anything but “crisp” sounding like that which you can get from a dynamic tweeter.  It’s laid back and mellow, and I think that’s what you’re hearing and doubt an amp will fix that because the Rotel is already a pretty detailed amp in the treble.  If anything the A21+ will likely be a step in the wrong direction as their house sound tends to the warmer side.  As a drummer I need to hear cymbal detail including the dynamic of the stick hitting the cymbal if it’s in the recording, and the Martin Logans I’ve heard just don’t give me enough of that.  They do other things really well, but that’s an area where they just don’t fit for me and may be what you’re experiencing too.  Just a semi-educated guess FWIW. 

What specific improvements are you looking for?