I have a jvc kw-av60bt. The head unit adds say it has a designated sub output but I didn't see it. Only the front/ rear, video and camera. The I have the mids and highs running off the front channel and the subs off the rear. The issue I have is my sub level on my head unit doesn't work. Whether I got -50 or +10 it stays the same. Any ideas what could cause this? It's driving me nuts.