I'd recommend you have it checked to make sure the power supply capacitors are still good. The large electrolytic caps in power amps often go bad after 20 or 30 years. The results can be devastating to speakers. This amp is at that age.
Hafler DH 500 update?
My brother just gave me an early DH 500. It seems to sound fine to me. It hasn't been used alot for many years so I am wondering if I should at least have it checked out and have the old board components upgraded or just go ahead and use it until something gives out. In other words, would I gain anything by premptive action?