It's I N C R E D I B L E... I was writing the same thread about the very, very interesting toberolling on the SRM 007t.
I'm testing from some weeks a lot of tubes 6FQ7 comparing the different sound I can hear to the SRM 717 SS and especially with my setup: R10/Angstrom.
As many of you know, my idea is to "tuning" the Stax system to the R10 combo, just to give a bit more of "musicality" to the electrostatic cans that is so much detailed and precise, things that would be against a certain pleasure in listening as the dynamics cans have more. And in this point the R10 has the first place between all the headphones (IMO and IME).
I can to tell sure the the RCA Cleartop shields are the most brilliant and detailed tubes, the second place we have the Ei, than the GE, and than others brands not so much good (as Golden Dragon, National Gold). But with the RCA the sound is too much "electrostatic", even if incredibly detailed and precise, I'm not looking for this with the Omega...
The first place absolutely for "musicality", we also can to tell warm and soul, are the Brimar/Mazda U.K... you can really listen the "triode sound", magic!
I just want to tell you my last experience with these fantastic tubes. This evening I replaced for another time the Brimar, a very selected quartet, I regulated near to the perfection the bias on the amp. (as Hirsch said there are some regulation (tvr-1 & tvr-2) not so easy to do... but I had a very good result.
Well, with the Brimar U.K. I have the incredible surprise: the Omega II is very, very similar to the R10, or better the Omega System is very, very musical with a beautiful intimacy, in the same grade of the R10/Angstrom. I was thinking... now I have another R10, but in a open version, I can to name them R10 II because the different is an upgrade of the R10 to a openest-clearest and very detailed cans.
I never experimented a so big different in changing the tubes like in the stax amp (system). And for this reason I prefer the 007t to the 717 because you can have the possibility to change the sound as you prefer.
The Brimar 6FQ7 are the only with a littlest glass, may be this is the secret... but I 'm not an expert of tubes... just I believe to my ears.
So I recomand the Brimar for the best performance of the Omega II. (I'm speaking considering my source, my cables, my ears and my comparison with the R10/Angstrom setup).
I'm still waiting for the last 6FQ7 quartet, the Philips (may be they are the Sylvania), but I think that would be very hard to have better performance than the Brimar/Mazda U.K: