I have noticed with my own amp, VK500 w/BAT pk. that whatever preamp or processor is attached, the amp does become compatible and takes on a new or different sense of performance. I also have not noticed any degredation of sound quality, sound staging, or timberal imbalance. The SS preamps as a rule do afford more depth of tome and impact in the lower regions and bring the stage forward more so... but that is just SS pres doing their thing. Running fully balanced or SE doesn't seem to matter in that respect as i've done both. A friend of mine has a vk200 which responds much like mine as he has done similar things with his.
on paper, you'd think runing all XLR is the ticket exclusively. It's not. I've noticed no drop in performance, nor have I noticed an increase in it either way. the sonic character is reflected completely, which ever way you go. SE or XLR. the big diff in sonic change with BAT amps that I've seen first hand is the things giving it the signal... and mainly, the preamps placed in front of it. always it has reamined coherent, musical, never strident or grainy. Never etched or two dimensional. Always smooth, dynamic, even tempered and resolving to the extent it adds to the conveyance of the experience and not to the point of distractive with a plethora of sonic cues. it's quite the chameleon... and has some pretty long legs as I've yet to hear it even approaching running out of steam.
I don't know much of the newer ones, the 250 and 600's. the only kick I have about the BAT vk500 amps is the time to warm up and sound good. 20-30 mins of warming, another 45-60 of signal passing till it's dead in stride. minimum toatls before being ready? 1 hour. But that's this amp and in my system and according to my ears.
have fun and don't worry... everything always works out.