I am using Alon I's (now called Nola) and a Rega Planet. The sound is airy, open, with excellent imaging.
I'm useing the B&W 803s with the KAV-400xi and it seems to be a very good combination (High-powered amp. w/High efficiency speakers). I find this Amp. to be very neutral as well as the 803s.
I've also heard this Amp. paired with SF Cremona, Krell Res. 2 + Revel F32, all combinations sounded very good to me, with the B&Ws edging-out on detail & imaging.
I have the CDM9 NT's mated with my 400 and for good recordings, the system sounds great. Poor recordings, not so much. You sorta get out of it what you put into it. Compressed rock sounds like....well, you can guess. Well recorded jazz, heaven.
I brought my amp over to my buddy's house recently and hooked it up to his 804's. That was a powerhouse combo. Glare free, taught base, smooth. We rocked the house down! He has a Denon receiver pushing his speakers and it is lackluster to say the least. I'd like to move up the B&W line to the 80x series myself.
I don't agree at all with the term "silky treble" to characterize the 400xi. Unless you have a silk-dome tweeter of course, then maybe you would hear that tonality. But thats not this integrated you would be hearing. Usually thats a term I would associate with tube equipment, and a few select SS integrateds; the treble here for the 400xi is well-defined, the mid-range is decent, transparency is there, fairly dark backgrounds. Overall presentation is typical Krell, dynamic, big and honest, lively, good to excellent bass control.
If I were looking for 'silky' highs, I certainly would not look to this piece for it. Overall this piece is a nice entry level, and an incredible value, and does all sorts of music pretty well. But I find it to be a little sterile and analytical for longer periods of listening, with little to no emotional/musical involvement. However most people looking for an integrated in this range may not be looking for emotional involvement.
I'm using my 400xi with a pair of Dunlavy Aletha speakers. WOW. I'm finally hearing what the Aletha's are capable of: Prodigious bass, smooth vocals, and pinpoint and non-fatiguing treble. It's still pretty new and the soundstage is starting to widen - can't wait for full break in! Can't believe it will get even better. I'm beginning to "live" in my listening room!
I use it with Wilson Sophia 1's. The result is dynamic and transparent sound, but slightly on the dark side. Its resolution and neutrality can not approach the KW500, but it doesn't sound as thin as the KW500 either. It has a nice warm midrange and is very pleasent sounding if you are not overdriving the speakers or the room. A great "bang for the buck" amp.