Way back early in this thread, Vinnie offered up some interesting options, one of which are the Mullard CV2492, which are also mentioned a few posts up here as well, I bought a matched pair from 1970 from Tubemonger.com I think I paid around $200 matched pair, maybe it was less, i can't remember. I own the Isabella and 70.2's LFP-V's, and found that these, for me, work very well in the Isabella. I use a pair of Mullard E188CC Dimple Disk GT in the amps. The sound difference is most affected when I tube roll with the Isabella, but I think that's typical for preamps. As you have the Sig's the differences would most likely be notable. The CV2492's are very warm, very deep, a lot of bass, which I like, and a pretty clear high end, but the highs could be a bit more up front, IMHO, (Dali's Helicon 400's and now Dali Epicon 8's), I think it's those ribbon tweeters. Overall, the Mullard's are something you might want to look at if you are thinking of a sound very different from most of the others i have tried, including the stock JJ's that came with the units and a pair of Tungsram PCC88's from 1971. I admit that all the hype about the Miniwatts has had me intrigued, but i haven't been able to find them at a price I was willing to pay, so for now, I'm comfortable with this setup.