Just a quick report on my impressions of these tubes. My Don Sachs preamp uses 2 pairs of 6sn7's, one in buffer, the other driver.
The Sylvania's have a lot of sparkle, In the driver position, they are great on detail but female vocals a bit grainy, maybe too much of a good thing. A bit lean on bass. If your system tends to the dark side, these would be a great match.
The best combination seems to be the RCA's in driver with Sylvania's in buffer. Great details and separation of instruments, vocals are smooth and sweet, bass is very nice. very holographic, the magic I have been looking for. Seems to be a good synergy.
Either combination is more enjoyable than the Shuguang's. Although they had a balanced frequency response, not much detail or depth with them.
Again, can't wait to try the Ken Rad's in combination with either of these.