Tube Preamp for Mcintosh MC501 Monoblock

Any recommendation on best matching tube Preamp for my new Mcintosh MC501 monoblock. Thanks in advance.
I have the 501's and have been using them with an older ARC LS2. I have since purchased a Mark Levinson 390s with a direction balanced connection into the amp with incredible results (makes me want to bypass using a preamp altogether). However, I do plan to upgrade my preamp as I am incorporating HT with my 2-channel (swapping IC's gets old after a while). So far I've have been considering the BAT VK-51SE and Aesthetix Calypso. The 501's are wonderful sounding amps, and having heard them without a preamp (depending upon what your goals are) I would look to find the most neutral and transparent preamp that you can afford.
WOuld there be impedance mismatch with BAT & CAlypso into a 10K input impedance amp?
Aesthetix recommends a 10k or greater load which the MC501 can manage (10k unbalanced, 20k balanced). The BAT has a very low ouput impedance (200 ohms). There should be no issues running either of these preamps with the Mac as it's input impedance is high enough to not load down either preamps output stage.
I use a Cary tube SL98P with my Mac 501s (driving an old pair of Apogee Duetta II's) and I am extremely satisfied. Very smooth, detailed; much better than the Audio Research LS2B I was using previously.
BTW, I A-B tested the MC2200 vs. the Cary and liked the Cary much better. Listen to a good recording of a horn section or viola and hopefully you'll know what I mean...
just heard an einstein hooked up to my 501s and am trying to justify the $$.

as a budget unit, the audio mirror unit is quite nice as well (needs a few $$ in parts upgrades though).