I know about the differences between the Cache DIMM and the ROM DIMM. The ROM DIMM has the Apple 341-0741 PN, cache does not, also the cache has 8 chips on DIMM, the ROM has only 4. So I know which is which

Yes, I changed both modules, also tried booting without Cache and with ROM in different slots, but I guess that the AV has bad logic board, since the CPU only gets slightly warm in 5 minutes, whilst the other one gets quite (normal) hot.

Also, the working one has a 820-0556-B revision logic board, the dead one has 820-0539-B, possibly any differences?