[Diabolical Artificer]

I'd put a grid stopper on each OP valve, something like 1k to start with, 100r for the G2's.

One other thing I've noted Joe is the balancing pots as shown on the schematic are a bit iffy, if a wiper lifts..... I put 1M resistors from the wipers on my pots in parallel as a fail safe. Here's two way of doing it below, see att. The second way can use a double linear pot, one pot connected "backwards", that way you only have one screw to fiddle with to get good balance. Wouldn't bother with the second pot, it's fighting against the first, would use 10r or 15r sense resistors to ground instead.

Lastly you'll have to go through some valves to get reasonably matched OP valves; you can find valves that are matched as regards quiescent current, but they won't be matched at full or even half power. I found valves benefit from being run in for a while before matching.

Andy.