I ordered a quad of Gold Lion KT66's. Quick swap, checked they were seated correctly and started the auto biasing off. A short while later there's a puurrumph sound coming through the speakers. I checked the valves and one, V4, was glowing red hot. The correct light was showing on the valve fault lights. I then swapped valves 3 and 4. Sure enough, same thing happened again but this time on V3.
I've put the original valves back in and I'm grumpily awaiting a replacement. At least I know now that the fault lights work ok, lol.