OK, here's what I've learned so far today, mucking around with this 7591 amp. I bought a matched pair of each of EH 7591A's and JJ 7591S's from ebay, hoping to compare the 2 brands. Note that all of the information below is only for 1 matched pair of each brand, hardly a statistically valid sample
EH 7591A's -
Cathode biased (at ~ 90% max. plate dissipation) - 17.0 WRMS max. output before clipping, but the waveform shows a lot of compression before this output, due to the cathode bias compression effect.
Fixed bias (at ~70% max. plate dissipation) - 19.1WRMS max. output before clipping, but the waveform shows almost no compression up to the point of clipping, due to the fixed bias keeping the bias voltage constant as plate and screen current increase.
The 2 modes sound very much as you would expect them to; more compressed (more like an EL84!) with cathode, cleaner, brighter and more dynamic with fixed, but a "sharper" overdrive sound. But I really liked both modes, these valves have a special clarity to their sound, and the overdrive is the sort that I like best, smooth with some grain, with not too much odd harmonic "buzz". Plenty of bass and treble, mids seem a little less prominent.
Problem was, there was a fair bit of audible hiss/popping sounds coming from the amp when I first put it together with the EH's. After ruling out pretty much everything else, I tried swapping in the JJ's. DEAD QUIET now. Hiss/pop completely gone, now the amp is super quiet. You hardly know that it's on until you hit a note.
But the JJ's definitely sound different (aside from the noise). More mids, less treble, less clarity, and they sound more compressed. They also bias up a bit differently, drawing more plate and screen current (3% more dissipation than the EH's in the same circuit). They also give 5% less max.clean output in the same circuit as the EH's.
Next step for me is to get hold of a 6.6K OT (like Ampeg used) to try these out on. I'm expecting some more output wattage with one of these.