K8060 expired just after powering up

I built a K8060 over a year ago and I’m very pleased with its performance. I recently completed building a second K8060, but this one has failed shortly after powering up, and I need some help.
The second K8060 was connected to all of the same equipment as the first K8060 which has functioned very well for over a year, so I’m assuming that these are not the cause of the failure. For the record, the transformer is a Multicomp toroidal 2x30V 160VA, the input is from a Velleman K8084, and the loudspeaker is JBL coaxial 4Ω.
The only modification to the standard kit was to upgrade C12 and C13 to 4700μF 63V (same as I did with first K8060).
Initial switch on with 60W bulb in the mains circuit was fine. Setting the bias was a bit fiddly, as at first it kept showing 0V until the adjustment was turned right up, but I did manage to get a stable 10.6mV reading eventually. After connecting the loudspeaker the unit worked fine at a low volume level for a few minutes. After turning the volume up to say 1/3, there was faint bang, some sparks and smell of burning, and the fuse blew.
T6 seems to have taken the brunt of this. The connecting wires are black, and the body of the transistor has fallen apart. The only other change I can see is the R2 has some discoloration in the centre, although my test meter is still showing 100Ω, so it seems ok.
I removed the new K8060 and reconnected the original K8060, and everything was working fine again with external equipment.
Could an incorrect bias setting be the cause? I notice the pot. is connected to T6.
Having looked at other threads on the forum, I’m guessing that the way forward is to replace all the semiconductors. Does this mean just the 8 transistors, or the 7 diodes as well? I’ve done a quick search round for the cost of replacement transistors, and I reckon it will cost about half the cost of buying a new kit (due having to buy minimum quantities, postage etc), so I’m temped just to buy a new kit. Or would you be able to supply a set of replacement transistors for this kit?
Many thanks for your help.