I can't help with the volume issues, but can say that series connected speaker setup is a pain.
I had two of the speakers dead, so no sound. I removed all factory speakers and went with two new 5.25" for the backbox and connected them in a new circuit with an L-pad attenuator. Less than $60 total and I have good speakers now. Still need to add a woofer and crossover.
Speakers still work via the coin door volume controls though, so whatever problems you have with that not working would still be an issue.
I rebuilt two of those sound boards this week, and working on a third now. There is a thread here somewhere with a parts list to rebuild a WPC nonDCS sound board. Replacing those has fixed the first two boards so far. The 4700uf Caps, tantalum caps, and voltage regulators are a good place to start.