I've found there is a fairly complicated path for sound power on System 3 games. Mine is borderline, seems noisy when the game is idle and in attract mode...but not as noticable in gameplay mode.
At first I also thought it was restricted to the cabinet channel and replaced a number of components including amp, caps and others in the output stage. It seems the noise is also present in the backbox channels, but "masked' by the characteristics of the backbox speakers. My ground mods are also done.
Due to the way the +/- 12VDC is created, split and trucked off the aux power board means there is no simple way to power the audio amps off an external supply. I basically stopped investigating the problem at that point. It just hasn't been a priority for me.

I'm not certain the noise is originating in aux power board in my game. There's all kinds of noise problems, and not all have the same root cause or symptoms. I'm thinking this might be a ground loop problem. If you replace your aux power board and that solves it, let us know. I'm sorry I don't have a solution at this time.

Thanks for posting.

Measure the AC ripple. If it is less than half a volt, I wouldn't bother.

Spectrum analyzer app pics - CBW

Game on (idle in attract mode)
28_Apr_2018_06_41_35 (resized).png

Game off (background noise)
28_Apr_2018_06_39_33 (resized).png

@woodmedic. did you do the ground mods? also carefully wiggle the connectors into the metal transformer plate. Sometimes these need repinned. I had a particularly bad one on the right side, after repair the hum was reduced dramatically.

@brunsie. Thanks for posting
that's a significant amount of troueshooting. I thought my noise was cab speaker only, it sent me on a wild goose chase of replacing components only related to the cab speaker channel. Upon further analysis, the same noise is on the backbox speakers as well, just much noticeable because the speakers there have different response characteristics than the cab speaker.
Just good for thought.

You can't technically repair any hum by speaker replacement. You can reduce it, because certain speakers have different frequency responses.

You have to find and eliminate the root cause of the hum or noise. A speaker replacement is a band-aid, a cover up, and the problem is still there.