There is no simple answer, but I would be highly suspect of simple statements that Krell amps are "bright" or edgy.
I have owned both the KAV-500i integrated and the FPB-600c. I found both of these amps to be spectacular sounding with my B&W Nautilus 801 speakers. I have now decided to biamp with tubes on the mids/uppers (mostly because the 801s let you biamp and I fell in love with the new VAC Phi 70s). I've concluded the following:
1. Boy, is the Nautilus 801 a great speaker. (But by all means take the grills off the mid and woofer).
2. The VAC sounds extremely fine up top, but truth be told, I am hard pressed to distinguish its sound from the Krell on the highs. That's right -- I am saying that the Krells do about as well on the highs as a push/pull triode 300B valve amplifier. In the crucial upper midrange, I would say that the two are extremely similar, except I find the VAC to be noticeably more forward than the Krell FPB-600c, although to be honest I can't tell you which one is more "right."
3. There is something emotional about tubes, but who can say what it is? As great as an FPB-600c is, it is hard to really love it -- it's a big, heavy, metal beast that sits there looking cold, impassive, monolithic and indestructible. It has seemingly limitless headroom and just makes everything about the music seem so effortless (until it blows your house fuses). On the other hand, the VAC Phi 70 is this gleaming retro object with evocative backlit meters and big, delicate tubes glowing serenely behind an etched glass casing.... If audiophiles were honest and admitted there is more to the audio experience than just the sound, perhaps we'd understand the romance of tubes a bit better! Maybe you should make your decision on which sci fi aesthetic you like better: if you're an HR Giger person, get Krell; if you like Jules Verne, get tubes! :o)
But from a sonic point of view? I don't see -- er, hear it.
4. Krells slow on the bass? Not the Krell's I've owned. Fast, authoritative and totally in control -- that's the experience I've had with these Krells. I don't know of any amp that does bass nearly as well, including Pass Labs and Levinson gear I've heard extensively.