Have you looked inside the amp? Leaking capacitors etc..? Might just need a good cleaning the 535 while the lowest watt was considered one of the best performance for the money at the time.
https://www.stereophile.com/solidpoweramps/adcom_gfa-535_power_amplifier/index.html
https://www.stereophile.com/solidpoweramps/adcom_gfa-535_power_amplifier/index.html