vk-50SE is THE BEST tube preamp I've heard. Staggering transparency with tube 'aliveness'. Most tube preamps trade resolution for this 'sense of life'. Not the BAT.
I use a vk-5i w/nos tubes with my tube amps. Less transparency, but golden hearted and with an exquisite midrange. I've put an ARC ls-25 mk I and an LS-5MkIII against it and it easily won.
I have not heard the new ARC Ref 2, mark II preamp. I owned the Ref1, and the vk-50SE ate it for breakfast, lunch, dinner AND evening snack. I did not hear them back to back, but the 5i might be better too.
Strangely enough, my favorite tube combination is the bat preamps mated to ARC vt100 or vt200. The two house sounds seem to fill each other in. I prefer it to the sound of the vk-5i/vk60 combo I heard. Sounded good, but the vt100 had more punch and this 'bold' sound to it.