Regarding your speaker issue, it may be annoying, but not a problem in normal use. You'll find ALL of the speakers are basically wired in series,
even if they use J504 and J505 separately. WPC games appear to use independent connections, but they are really just using two pins: J504-pin3 and J505-pin2.

See http://www.dziedzic.us/wpc_speaker_sound_info.html#newwiring for schematic of how WMS machines were wired.

The series thing simply helps understand WHY disconnecting ANY speaker in the machine (the two in the backbox or the woofer in the cabinet) will yield NO sound at all. As for the screeching and pops, that is a variation of a litany of similiar issues everyone has with the early WPC FH machines. If the capacitors all check out, and the power supply is at the right levels, then I think you'll just need to live with it or add a mod. (In 2005 I had purchased one of the power pop modification boards which basically disconnected the speakers during power-up AND power down, essentially camophalging the design issue. Ironiclly I just sold it a month or so ago... but there are others with similiar mods floating around.).