Some info
here.
See the Interstage Tranny post. What's interesting is he rates 6A3 (doesn't mention the maker) above it, which I initially did. However, I changed my mind over time.
All bets are off if your VT-52 isn't exactly the same as the ones I have posted i.e. NOT Sylvania and NOT with the same internal construction (well, the later is the most important aspect, obviously

).
There are higher voltage WE VT-52s on ebay at the moment for lotsa money.
It's well made and low microphony, much less than a KR PX4, say (dig). However, I am expecting the KR 300B RK to be low microphony, since it felt much better made than the PX4 in the hand.
Unfortunately,
www.vt52.com is down.