Haven't seen this asked yet, but where do you have the gain set at? I know that sounds so stupid, but I've worked in install bays before and that tends to be one of the most common reasons for this...just turn it down a notch, usually does the trick.

I still cannot tell if the subs are wired in parallel or series. It depends on how the wires are run between the amp and the subs itself. If they're "daisy-chained" together, then it's in series and you're running a total impedance of 8 ohms, which should be fine. If they're wired in a separate manner, then it's in parallel, which should give you a total impedance of 2 ohms...that would be your problem if your amp is not stable at 2 ohms.

Hope this helps somewhat.