IMHO, the KSA-Xs series was anything but bright. If anything, these amps shaded very slightly darkish to my ear. I owned a KSA 50s and auditioned the bigger models extensively, and I might have sensed very slightly closed, diminished treble. It's true that these tended amps also toward powerful deep bass and that they could be a little lean thru the mid-bass. The net impression for me was slightly "thin" or "analytical", but overall pretty darn neutral and definitely not bright.
So, for a matching pre:
I'd agree that the balanced output is a good idea and also agree that tubes work well. I used a SF Line 3 with real success and ARC LS-25 (leaner sounding, itself) somewhat less successfully. I also used a Audible Illusions Modulus 3 (single ended) and I liked that match a lot - even it wasn't as quiet as the balanced preamps.
Bottom line: a characteristically "tubey" sounding pre might be a good idea and a balanced out would help, too. Based on my experience with their Rhea phono stage, I suspect that the Aesthetix Janus or Calypso linestage might be a good call. However, I haven't directly heard the pairing, so this is speculation on my part.