Well, in my opinion the Japanese X386S (if you can find them, and.. really, they are big) are probably the equal of the Black Gate WKZ, in circuits that I've tried them (PSU). At this voltage, they are probably one of _the_ caps to beat. I dunno about the cathode bypass caps, as I avoid them where I can and where I cannot, I have pretty small voltage and have found polymer caps to work better than anything I've tried. I don't really have the experience that Doug does here, so I'm sure whatever he recommended would be worth trying. I will say if I had to just try something, since I don't have much experience with this amp, I'd start with a Mundorf M-Tube Cap 47uF/600V or 30uF/600V and bypass it with something like a Relcap RTX.