Folks, I realize the OP's question has been answered, and that he made a decision to go with a Parasound A23. I figured I'd resurrect this thread because I've been considering an A21 and XPA-2 myself, and I saw no reason to create a second thread with an identical title.
I currently am using an Emotiva UPA-2 to drive a pair of Ascend CMT-340 SEs. My rig also includes an Emotiva XDA-1 DAC and a dbx 223s active crossover (which I use to send sub-65Hz signals to a sub). These latter two components have balanced circuitry, but the UPA-2 amp is unbalanaced, so the system currently runs unbalanced. I've read in several places that switching to a balanced systems offers noticeable improvements to sound quality, and I'm very tempted to try this. At the same time, I'd like to increase my headroom. Both the A21 and the XPA2 would accomplish these two feats. Of course the A21 costs more than double.
A few of the responders to the OP mentioned that the A21/A23 sounded much better than this amp or that amp that he/she'd previously owned, but I didn't see anyone who'd directly compared the A21 to the XPA2. Has anyone heard both? Did the A21 sound better? Are you able to describe it?