Have you tried 5751's in that? ECC83's aren't great in the circuit imo, particularly the third tube doesn't seem great at driving interconnects. 5751s are a slightly lower gain, drop in replacement for the 83, it'll sound slightly different at first but when you get used to it you should find improvements all round. There are some nice GE ones at Watford but they're £18 a tube or something daft. They do some others around £9 each (Phillips or something, can't remember off the top of my head) which are also very good.

I'd have thought they're worth trying in any amp using 83's really, which I think is better suited to guitar amps than hifi having a bit too much gain for my liking.