Thanks again, folks, for your excellent suggestions. I liked the 508ia right out of the box, but after 40-50 hours, I just wasn't getting the refinement I wanted, which I (correctly, I think) attributed to the stock Electro Harmonix 6SN7 and Sovtek 6SL7 tubes. I replaced with those with a pair of Sylvania JAN VT-231s (good call, Wig!) and an RCA VT-229 and was astounded by the immediate level of improvement I experienced. I don't want to fall prey to recency bias, but I do believe that the 508ia is the best amp I've ever heard in my system, better even than the VAC Ren 70/70, which I dearly loved for several years. I am not sure that the 508 has quite the same degree of mid-range magic that was the VAC's strong suit--though with the NOS tubes, it is at least a horse race--but the bass is better (deeper, more clearly defined, etc.) and the musicality is a draw.
Of course, the usual caveats apply--IMHO, in my system, YMMV, and so forth. The really exciting part is that I might have a hundred hours on the amp max, so it is probably still breaking in, and I have not yet rolled any 805 or 300B tubes to see what further improvements that might yield. I will say that I could happily live with this amp just as it is for the rest of my days, and that to anyone thinking of an audition, find, beg, or borrow some NOS tubes so that you will know what this amp can really do. It is just spectacular!
I will certainly look into the fuse upgrade, and those Psvane sound like a no-brainer at $125 for a matched pair. I had an octet of KR 300Bs in my VAC Ren, and I hate to confess what I paid for those.
Finally, this amp is just beautiful to look at, especially at night!