It's a bit amazing just how tenacious the SP curse is, even my 100% rebuilt amp hasn't escaped it completely. Just about the only SP item I reused was some of the old wiring for the filaments. The only reason is that it is rather large and being solid core, can be twisted tightly and once fixed in place, stays there. The problem is the insulation though, it melts and splits even at low heat.
So one of the times I fired up the amp this week I cued up the first song and half way through it got quieter and quieter until distortion crept in. Hmmm, I looked at the amp, all leds lit and so were the 7n7's but the EL34's were dark. All voltages tested just fine on the PSU so it had to be the fubar filament wires yet I found nothing wrong with them or my soldering. I moved the wires around a bit and the problem hasn't resurfaced in the roughly 40 hours the amp has been in use since then but the wire is clearly cursed...