I'm about to trade up to an NAD T763 after owning a T762 for a little while. The only problem has been the documented "digital signal lock" issue - do a search with T762 and signal lock and you'll probably find it (do it here and at
www.avsforum.com in the audio/amps section).
Doesn't really bother me though because I use the DACs in my CD player and my DVD/DVD-Audio player (and when I get a combo SACD/DVD/DVD-A player I'll have to use those DACs). But if you're going to use the DACs in the T763 for CDs it might be an issue. I tested out using the DACs for movies (optical out from my RP91 to the T762) and the signal lock issue isn't an issue.
I'm upgrading simply because I have a good deal from my dealer since I recently bought the T762 and the T763 just came out right after my purchase.
I have 4 ohm speakers (MB Quarts) which choked and nearly overheated my old Sony STR-DA555ES. I was so surprised to hear how the T762 drove them. I re-discovered CD music. It's amazing how much of a difference that real power (vs. manufacturer's ratings) makes. I'm sure the H/K (which is well respected here in HTF) provides real power too, but not having owned one I cannot give you a head-to-head evaluation.