Help please, one monobloc amp running hotter than the other

I have a pair of Denon POA-S10 mono amps from 1995 and they have been trouble-free until now. One amp is running noticeably hotter than the other even at an idle state, after playing a full cd at moderate volume it gets really hot while the other amp is just warm. Already double-checked all connections. I thought it may have been a cooling fan that went out but after I opened it up there is no fan, just a massive heat sink. I am running balanced inputs from a PRA-S10 and both amps sound as good as ever, there is no sound problem. Also, the hotter amp takes longer to start up, there is a faint relay sound a second or two before the louder relay click that tells you it is ready to go. The cooler running amp just has the louder relay click and starts up in about half the time. They used to start up at the exact same moment from being powered on in about 3 seconds.

Any ideas on what could cause this would be greatly appreciated. If you need any more info please ask.

Thanks