You are over thinking it. The L-pad is going to vary the impedance anyways. I did this on Stern Harley Davidson, but the backbox speakers are wired in series. So I was able to balance the volume level between two series wired backbox and the mono cab speaker, except that I had already replaced the cab speaker with a subwoofer speaker lying around, which the games amp could not drive very well. I added a speaker-to-line level converter and separate 100w amplifier board to the cabinet. Now it's excellent, I can dial in any levels I want.