Headunit or WS420 problem

I have an Alpine CDA-9886M head unit and the older version WS420. At the end of the season last year when I would link to another boat through the boat link on the 420 my head unit subwoofer eq would automatically drop to -4. After adjusting the eq on the head unit back to flat the volume of my stereo would be cut in half even with the volume all the way up. I suspect the problem is in the head unit but wanted to hear from the experts what you think and if there is a way to test which one the culprit could be or both.