I have never heard A21... But I currently do own a 23 ...and a few amps from Emotiva. I still can't believe Parasound could put out such a POS. The bass is bloated, mids are boring, highs are just not there - this amp steals half of the HF information. I\ve tried it with a few sets of speakers ( Martin Logan Motion 12, Polk Audio LSi15, Energy Veritas 6.2 and some huge JBLs ), a few preamps both tube and SS , lots of power, speaker and XLR-RCA cables and the verdict remains - it's just a very bad amp. The sound is tied to speakers, the soundstage is next to non-existent. Even XPA-200 ( retailing for 499.00 ) kicks this Parasound's butt around the block. XPA-2 is in a totally different league. I am trying to get rid of the A23 but when people come to listen to it they are not exactly carried away by the sound it outputs, especially after listening to my Emotiva gear.