I recently read in the 6moons review of the Meier Concerto that Jan Meier described the AD797s (which were used in both his Concerto and the current Classic model) as being particularly sensitive to power supply quality:
Quoting Jan Meier:
More than any other opamp I've used before, the AD797 is very sensitive to power supply quality, hence I run 85.000uF of capacity via Nichicon audio-grade capacitors as well as extensive regulation and filtering. I always use linear power supplies to prevent high-frequency noise pollution.
This gave me revisit my use of AD797s in the buffer stage of my PB2 (where I had previously reported extreme overheating during an evening's use, but have not been able to reproduce the overheating since then).
I had a vague recollection that I may have been using my Energizer XP8000 external lithium-ion battery pack to power the PB2 at 16VDC, while the XP8000 was being recharged - which normally isn't the case.
Sure enough, the AD797s started overheating when the PB2 was powered by an XP8000 battery pack that was itself being recharged via its switching (non-linear) AC adapter.
When I unplugged the XP8000 from its charger, the AD797s went back to normal operating temperatures (they stopped oscillating).
Mystery solved. The XP8000's charger can cause the AD797s to oscillate.