Wire both speakers to one chnl and run it for the day. If it still pops the code, move both speakers to the other chnl and run again

Are you absolutely positive its set to hi-pass? Full pass can over drive the speaker, dropping its impedance low. However, since you are running 4 ohm, you should have plenty of head room impedance wise. But always worth double checking the cross over by moving, then moving it back just to feel it drop into that HP detent.

id also want to monitor the amp's supply voltage while under load and even better, when it goes into protect mode.

Moving forward, id do away with the pass-thru RCA. Get a good hi to low, like the Kicker KISLOC, and run the in-boat/woofer amp on that other zone.

Head unit EQ settings?