Thanks for the post, Cat. I was feeling down in the dumps about not being able to use the tubes and not being educated enough yet to know that they could damage my precious 500-C.
I do have a digital EQ as my signature indicates which is applied to all my signals. My signal path is source device > patch bay/mixer > digital EQ > Fisher 500-C > AR-2Ax speakers + subwoofer. The exception is when I play directly into the AUX input on the Fisher such as with a tablet and phone (both running PowerAmp as my music player). I've started connecting my turntable into the PHONO input on the Fisher which sounds pretty good too.
Anyway, I am researching the 12BZ7 and 12AX7 tubes so I can learn what elements of the BZ7 make the great sound I like (it's not just EQ - there was an overall and significant "umph" of some sort) so that I can look for those elements in other tubes that might be compatible in the 500-C. Or maybe there is some alternative way of using the BZ7's that won't "take down" the SDS board or power transformer "long term". I don't know. If I don't learn anything applicable, I may just resign myself to the fact that the 500C already sounds fabulous as is. I mean even artists (and I'm one myself in the world of music composition) have to know that at some point enough is enough and it's time to start the next project.
I see that the BZ7's transconductance, plate resistance and plate current are twice that of "normal" AX7's. Interelectrode capacitances are also quite different. I don't know if I can electronically "normalize" those specs, for example, or if I can do some kind of add-on transformer to provide the extra current the BZ7's need. From my reading it seems I can add some kind of resistance in the circuit, somewhere just after the power transformer, that satisfies the "thirst" of BZ7's. But even if that worked, I could be ruining the elements of the BZ7's that make it so much to my liking. I'll keep looking for an answer...for a while anyway. Maybe just a pair of Genelex Gold Lions.
PS - I went browsing through my local record store today and they have a 500-C with the older "large numbered" FM dial on it. It is in terrible shape with knobs falling apart, switches that feel like they're locked, just in real need of some work. Price tag? $700 !!! Wishful thinking !