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.