I never especially liked how the stock V1 tube sounded. Don't get me wrong, it was OK, but I'm shooting for blows me away. I keep a small stash of replacement preamp tubes on hand, both NOS and new build. After much experimenting, I went with an NOS Raytheon 5751 and upped the gain control correspondingly from 5 to 6. As always, of course, it's a personal decision and YMMV.