Is my Amp OK?

Yep! that's exactly right. I don't know what I don't know. There are known unknowns and unknown unknowns :-)

While it sounds good, I wonder if there is anything I can make it sound better. The everlasting quest for holy grail!

We don’t know, what we don’t know...meaning the only way to find out if your amp is okay is to have home auditions of other amps. I love my amp and preamp also, yet I’ve been going back and forth asking myself if I can improve my current amp and preamp within my allotted budget. I’m going to get a home audition of a couple amps/preamps to answer my own question.

You say you "love the sound." Why, then upgrade from a Rotel RB-1590? Maybe a case of "upgradeitis?"))