You can try to disconnect the existing 680uF caps - one may be leaking or otherwise damaged. Use only the single 6800uF cap.

 

Hello,

At capacitor C757 there will always be hum.
Have a look at this picture about the effect of the capacitor:

More info in the attached PDF.
It is more a concern how the ouput of the speaker is sounding.

Bertus

 

Thanks for the PDF. A great resource for beginners like me.

The amplifier sounds good, the sound is nice and warm. The problem is hum that is masked when listening to music, but when the tone is significantly or completely reduced or during song breaks, it is very pronounced.
I would like to eliminate that (if possible).