Issue with crackling channel on bi-wired MX50 - CX50 Yamaha amp set

Hi!

New user, glad to be here 

I have an issue with a crackling/distorting treble in the left side that I hope you can advise me on. It's like a white-noisey scratchy sound and the treble is also not constant, but going on- and off in a "blinking" kind of fashion.

I delivered the set for repairs and just got the amp set back "cleaned and balanced" from an electronics repairman, but the issue still persists. I reversed the speakercables and the crackling then changed to the other side as well. Can anybody say with certainty what is wrong here and what needs to be done? The meter-columns on the front side showing channel load also still don't go equally high up when showing load, so that didn't improve either.

Thx for any responses.