Marshall mosfet 100 - power amp issues

[Enzo]

There are even dual TO92 heat sinks for pairs of transistors, like the diffy pair in an amp.

A dab of silicone will keep them in place.

[Dave H]

...anyway the board is not brown, and the amp has been working fine for many years.

I think it will be OK. There are two de-rating numbers for the MPSA06 in the data sheet and it's running well below the max permitted dissipation using either number.