I've listened to the preamps for about a week now, and what I hear closely mirrors what both tomic601 and almaaudio have already offered. The difference between the two are not subtle.
The Ayre KX-R Twenty is as close to the seemingly ideal "straight wire with gain" as I've heard. But the Ayre in stock configuration is basically an active attenuator. It only offers about 1-2 dB of gain, wide open. Different resistors can be swapped in if more gain is needed. Its presentation is clear, nearly transparent, with drive and focus. I'm not sure I could pass a blind comparison between having the Ayre in circuit vs without. The biggest differences, and the differences are slight, are more highly focused images thru the Ayre with a very slight loss of richness. Someone looking for a Very transparent preamp should hear the Ayre. The chassis is a metallic jewel. Beautiful.
My first impression of the Audio Research Reference 6 was Wow, its Big. It physically will only fit on the top shelf of my equipment rack. And it needs room to vent heat. Its presentation tends to highlight the midrange, not because the bass is reduced, and not because the highs are rolled off. They are not. The three dimensional richness, especially heard in the midrange that is conveyed is undeniable. The bass is solid and strong. If the Ref 6 has a defect, its in the treble. Most easily heard with piano, it lacks a bit of realistic sparkle in the upper registers. This can also be a blessing with some hotly mixed digital recordings. Almaaudio said "air, soundstage depth/width, presence". Yep, that pretty much nails it.
It comes down to do you want a preamp that very nearly succeeds at being completely transparent, the Ayre. Or do you want a preamp that adds to the presentation some really musical things, the Audio Research.
In my system, the decision is easy. If I want total transparency, I will drive the amp directly with my Berkeley. If I want the merits the Audio Research adds to the soundstage, I'll run it active.
And I think I might be able to improve the high frequency performance of the Ref 6, not that it needs much. I owned a Reference 75 SE amp about 6 months ago, and I was able to improve its treble some with a Nordost power cord and a Synergistic Research Quantum Black fuse.
