This is comparing the sound of the systems probably more than anything, so take it with a grain of salt. I think the A21 is probably a little more refined than the 3500. However it (or system it was in) did not have the presence of the 3500. midbass punch and bass depth, i am guessing goes to the 3500. i would class that area of the 3500 up there with the Krells I am using now. Its very hard to compare these two amps when I did not have them in my system together, like I did the rotel and 3500.
Downside of the 3500 is it is two amps in one box and uses two power cords. It needs power from the wall. Sound is big, detailed, fast, and powerful. midrange is detailed and clean. Built like tanks. I have also owned the JC-1s and didnt really find them much of an upgrade over the 3500. They did get rid of some grain from the upper most midrange and treble, but didnt have the weight behind the music that the 3500 did. I used the 2000 preamp with the Rotel, 3500, and off and on with the JC-1s.