I have heard them both on my Apogee speakers. The BAT has a detailed presentation (like Larry said). The brighter side of neutral...overall a very nice amp. (I could live with it).
The Krell has a somewhat lay back sound...the softer side of neutral, (I did live with it). The Krell has more power to drive crazy speaker loads...Apogee Scintilla, Infinity Kappa come to mind.
I don't agree with (Samzx12) regarding the Krell FPB 200, although...the FPB 200 is more "BAT like" in it's presentation. So....I can understand someone liking it better.
The big Krell is old, will need rebuild, runs hot, is around 50 lbs more weight, needs a dedicated 20 amp circuit, shipping is costly, sounds best run balanced, and....needs a preamp with lots of gain to bring out it's best.
In todays world, I would probably go for the BAT over the Krell...even though, I liked the sound of the Krell better with "my speakers".
Dave