Hopefully this helps someone in the future.
My 500V offboard KGSSHV (running at ~10mA, 100R for the current adjusting resistors) developed some grainy distortion on the left channel (only with some low bass reverb frequencies). After replacing all sorts of parts, I found that the distortion was due to the IXYS transistors.
I had bought 2x IXTP01N100D from Mouser in June 2020 and 4x in Jan 2021. It turns out both of these sets were "bad" parts, symptoms below:
1) Offset voltage on left channel started at around +35V and drift upwards to around +70V
2) There was no way to lower the offset voltage, as the trimpot was maxed out
3) The sound was a bit metallic from the left channel
4) The volume was a bit lower than normal
Fortunately, spritzer sold me some of his old stock IXTP01N100D, and after replacing the new IXYSes with spritzer's old stock IXYS, all of the symptoms went away, and the amp sounded "normal" again.
With the old stock IXYS:
1) My offset voltage started around -35V and then drifted towards 0V
2) Amp seemed cooler to the touch
BTW, I think the 500V KGSSHV offboard at 10mA is the best pairing for the 007 mk1.