Most Musical 6NS7 tube for a Preamp?

My vote is for the Ultraverve also.  I have had mine for about 12 years and, as a result, have tried an amazing number of different vintage 6sn7’s. There are at least a dozen really good ones...each with their own personalities.  That said, each of these tube are all very relative to your particular rig configuration.  I run my Ultraverve through a big Bryston 14bsst ss amp and then on to B&W 802’s. As a result, my favorite 6sn7’s are what works for my system...and to my ears.  I will say that swapping out this particular tube in the Ultraverve can make dramatic tonal changes in the sound of your system. As Kara Chaffee describes it, experimenting with different 6sn7’s is akin to trying different wines! Also, the Ultraverve is built like a rock with point to point wiring, choice of remote or not, and designed to put very low stress on the tubes. I had Kara raise the 5aw4 tube seat to accommodate a bulbous 5au tube. She also updated my Ultraverve with Dueland caps and wiring about three years ago.  It sounds amazing and I get the best of both worlds, the warm musicality of the tubes along with effortless power of the sis amp.