Help! Trying to Make Kenwood KA-3500 Subwoofer Capable

Hi everybody, as the title says I'm in a bit of a pickle. I've been using the B channel (converted from speaker wire to RCA) to go to a Sony powered subwoofer (SA-WMS 230). I have the 2 outputs (L and R on B channel) connected to a stereo to mono converter, which goes to a rca switch to switch between a Sony Str 501 for subwoofer connection. The problem is the output to the sub is very loud, and all of the signal range, not just the low signals for bass, when switched to the kenwood input. The Sony amp signal is unaffected, as it was before I rewired this whole thing.

The a+b switch works on the kenwood, but causes the a speakers (Kustom KSC-10) to play very quietly. I'm not sure what to do here, I don't know if adding a low pass filter between the stereo mono converter and the rca switch will work, or if I'd be able to use the filter integrated in the Sony Str 501? Not sure if I can send the signal there, as it's already amplified. Has anyone ever tried to do this and gotten a good result? Is too much power from 2 channels going to the subwoofer? Or am I just barking up the wrong vintage audio to modern audio equipment tree? Any help would be greatly appreciated! (Please ignore any messy wiring/things not being plugged in).