HK Lucas Max amplifier hiss

Hi all. I've recently acquired a HK Lucas Max system but am experiencing what, to me, seems like far too much background hiss (also also the occasional pop) through the satellite speakers, even with no audio source connected. It's definitely more pronounced on one channel than the other, and stays with that channel even after swapping the sattelites around.

I haven't owned one of these systems before but I don't think this can be normal, as it's definitely noticeable in a quiet room. The background noise level is the same regardless of where the gain controls are set, but it isn't noticeable with music playing. It's as if the noise floor is higher than it should be.

The system is quite old and the amps are class D, and I know PWM amps can be a bit picky with power supply ripple, so I thought perhaps it might be worth changing the reservoir caps on the low voltage side of the SMPSU. However, they show no obvious signs of damage (bulging etc), and I'm not keen on replacing parts willy nilly on the off chance it might cure the fault. Finding caps with similar dimensions and ripple current ratings to the original Rubycon USR's is also proving difficult.

I've heard people suggest cleaning or even replacing the gain pots, but they seem to control the gain just fine when in use (the noise isn't noticeable when music is playing), and looking at the service manual, it looks like the pots only feed the ADC of a microcontroller in any case. There's no line level audio going through them directly.

I'm going to go over the entire amp PCB and resolder any suspect looking joints, but beyond that, can anyone with experience repairing these amps point me in the right direction?