BAT or Sonic Frontiers w/ Martin Logans?

I am using a BAT VK3i with Magneplanar 1.6QRs, so while I can't address your specific match, I can tell you that this is a very neutral preamp. I don't notice any particular character, other than the fact that music from my system just seems to wash over the entire room. FYI, amp is a BAT VK200 (solid state).

I'm using a VK-3i with my Magneplanar 1.6QR's also. I found that the preamp benefits from using good NOS tubes - I settled on Amperex Orange Globe 6922's after trying various brands and substitutions. The 3i is very neutral and sounds great but I wouldn't describe its tonal balance as being "warm".

I also recommend the Mod3 which I've been using for 7 years now. Great phono section for the $$ if you need this. Two points, though: (1) Get rid of the Sovteks. Out of necessity manufacturers have to source tubes that are readily available in quantity. There are lots of NOS tubes out there that will sound better (using Seimens 6DJ8s)and let you "tune" to your tastes; (2) Be aware that this pre has incredibly high gain. Not a problem with some amps but can be frustrating with high gain amps. Not a sonic problem, but limits fine adjustmant volume and balance. Good value on the used market and with only 4 6922/6DJ8's you can afford to experiment with higher quality NOS tubes.