Hey Berndt: So glad you like the Ultra. I think Lloyd's products are some of the most underrated and great values in audio today. Don't know if this will be good for Dr. Peppard or not, but the word on MAD products needs to get out. Anyway, I haven't done a tremendous amount of tube rolling on the Ultra, but will briefly report on some findings (assuming my sonic memory is accurate). Again, the usual caveats, my gear, my room, my tastes apply here. There were some NOS GE 12sn7gts that were in the line stage that I replaced with some NOS RCA 12sx7gt and never looked back. The RCAs seemed like solid well balanced performers and I had no need to look further (and was having trouble looking anyway as I couldn't find all that many 12sx7s that I thought would equal or better the RCAs). Also found that the rectifier tube made for more dramatic changes. I currently use a Mullard GZ35 in the 6x5 spot. I have tried a Bendix 6x5 which I found charmingly thick and romantic, but was way too syrupy for my taste. I also tried a Sylvania 6x5, coke bottle shape, which was way cool looking. It was a well balanced tube from top to bottom. Definitely more energy on top than the Mullard. Ultimately, it did not have the midrange warmth and 3D bloom of the Mullard so it did not stay in my system all that long. There was another one I tried, a GE I think, but for the life of me I can't remember. Must not have made an impression. So far, I have liked the Mullard as the rectifier tube the best. Hope some of this helps. Keep on enjoying the Ultra. Its a great procuct and happy listening.