So, a few weeks ago, on perfusionist's recommendation, I bought a matched quad of beautiful blue Sophia Electric El 34's for my GA TRP. I now have about 175 hours on them. I must say that I am very pleased as they sound fantastic.
I was originally looking into NOS GEC KT66's but was choking on the $1400 price for a matched quad and the uncertainty as to whether the ones I found were in good shape or even the version many are raving about. At the time, I had recently heard a loaner tube integrated in my system while my Ayre AX-5/20 was in the shop for a repair. While it lagged far behind the Ayre in most areas, it did have a drop dead mid-range bloom that sounded great with vocals and acoustic instruments. It exposed my system as a touch dry and sent me looking for a path to add some liquid to the mids but without losing the balance, high end extension, dynamics, and punch of the Tung Sol 6116s that I had been using. That got me thinking about new tubes.
As to the Sophia's, they are exactly what I was looking for. The mid-range is absolutely beautiful and vocals and acoustic instruments sound both gorgeous and natural. Not "bloomy"as such, like the tube amp that lead me down this path, but still very liquid, rich, and engaging. And, they retain the other great characteristics of the TS tubes I had previously been using and enjoying.
I have not yet heard the NOS GEC KT66s and I may never do so given the price and scarcity. That said, based on the descriptions I have read of that tube, I feel like the Sophia's must be very similar in quality and character. All for about 1/3 the price. Highly recommended.
FWIW, I have also enjoyed current production GL KT 66 and 77s, and EH 6ca7s. In comparison to the Sophia's, however, the KT66 is linear and balanced but a touch dry. The 77s are fantastic-punchy with rich mid-range, but were fatiguing to me after a long listen. The EH have a fantastic liquid mid-range, but lack high end extension., While I can imagine audio peeps enjoying each of those, they are not in the league of the Sophias imho.
Oh, and thanks so much perfusionist for the recommendation.