valve experts - 6sn7 suggestions?

I also have a VAC 30/30 Mk V with the Signature upgrade, and have found the Sylvania's to be a very good choice. The VAC amps run the input tubes and driver tubes at very high plate voltage (over 400 volts) and you should be using a Sylvania GTB 6sn7 which are rated at 450 volts to be on the safe side, especially in the driver tube positions, which are run a bit hotter than the input tubes. The best sounding though is the VT231 which is not a GTB tube (it is rated at around 300 volts) It is common for some to run the VT-231's at these higer levels, but I had to go through a lot of 231's before finding a pair that were able to operate properly at these levels. The next best sounding tube is the late 40's early 50's GTB's. The input 6sn7's have the most dramatic sonic signature, where the driver tubes are not quite as obvious. The Sylvanias will have much better dynamics and bass impact compared to other NOS tubes I have tried. The original VAC tubes however, sounded better than all of the NOS I tried except for the Sylvanias. Kevin and the crew at VAC did a wonderful job of balancing the sound of the amps with their original stock tubes. The Sylvanias may be a bit more forward for some tastes in the mids., but that of course can be very system dependant. Hope this helps!

Erik;
A real learning experience about the 6sn7's will start by paying a visit to Andy at V.T.S. (Vintage Tube Services)
He has a nice web page and is the authority when it comes to tubes of all sorts. He will soon be writing a book on tubes, and it should be one of the best around. He will straighten out any confusion about the voltage ratings and what you might need. I tried numerous Ken-Rads, Tung-Sols and Sylvanias from him, and it was an enlightening experience.
He has a great return policy, and will give you plenty of time to try out various ones. The Sylvanias have much lower
microphonics than the Ken-Rads and Tung-Sols, and are less "colored sounding" with the VAC amps. Happy tweaking!