Power amplifier repair

I bought a vintage citronic ppx900 power amplifier one channel is ok but there is some sound problem on 2nd channel when i opened the amplifier on that is channel there were 3 pairs of k176/j56 and 6 fuses at each fet drain.1 of that fuses was blown up. I checked the fets.1 of that fets was also not working.than i disconnected the a whole pair.Now only four fets are working. Amplifier is working ok but when i increase volume of this channel i feel whizzing sound.
Any one knows what the problem is ?
Also want to mention that before disconnecting blown fet there was more sound problem.After disconnecting the sound was clear but not 100% .With increasing volume disturbance also increases.