HELP! Having trouble with Alpine HU and Sub

I just got done installing my Alpine cda-7875 HU, and there seems to be a problem with the bass. It's on ALL the time, real low and mellow like...and it doesn't change, no matter how high/low you set the bass. I checked all my wiring, and it seems to be correct. It's got 4V pre-amp output for the subs. The amp is a fosgate 250a2. I was using it before with my stock HU and some of those PAC controllers, and everything worked fine. Now, I am using RCA's straight out of the HU back to the amp. Everything for the amp is hooked up correctly. Any ideas what the hell could be going on? Once again, even when the volume is at 0 on the HU, there is a very low mellow hmmmmmm coming from the sub.
HELP!!!
Everything else works fine, incl. head unit, and all door,rear speakers.

Sorry about the double post...Didn't think the 1st went through.

If you unplug the RCA's from the head unit, does the sound go away?

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