Wow, nice solve....eventually.

I think you mentioned earlier that the hum was dominate more in one speaker only?
I wonder if maybe the speaker was suffering a partially burnt coil form being overdriven too much (previous owner) and possible making it a little more resonant at low frequencies, especially 50Hz hum????

Also, i wonder if the original speakers are maybe generic audio speakers 20hz-20khz, and your replacements might be generic mid-range speakers for the 100's Hz upwards scale, therefore not being too resonant at low frequencies like 50Hz hum????

Also, the new speakers are now mounted on a spacer/washer/, i wonder if that's helping to isolate hum from the speaker into the spaeker-panel too??

Quite possibly the supply of cheapy generic speakers has been the problem right from manufacture.

I would have thought to use mid-high Freq speakers up top, and keep all the bass stuff in the cabinet bottom driver.

Nice find, glad you got thru it ;-)
MM.