Revox help

My B-I-L has a Revox integrated amp (45w/ch) that he bought new in 1984. He has develpoed a problem that I can not diagnose. He seems to have a 'crackling' in one of the channels, or so we thought. When he reversed the channels (amp has a reverse channel function knob) the crackling was coming from the same speaker (right speaker 'A' pair {ADS L-710}). Ah ha, so it is the speaker you say. Problem is it is also making the crackling sound from his 'B' pair right speaker, even with the 'A' pair off. Since it is not channel-relevant or speaker-relevant what could it be? I am baffled.

Interconnects would indicate channel-relevant as would speaker cable.
a)power supply
b)rail caps and many more electrolytics had ended their life which is typically less than 20 years.
problem in power amp. right chan is always rt. chan. regardless of settings, outputs etc..