Shiva-x + O audio 500w = extra boost at 30-35hz

I have been messing with this combo since day 1 of the build (4 months). Right now the shiva is in a 3 cubic feet sealed square box and it has what it seems to have a extra boost between 30-35hz. I have the O audio 500w plate amp set to -12db in that area but still during movies (DTS and DD) it likes to over-extend a tiny bit. If I do not set to -12db the sub really starts to make noises and seems like it want to come flying out. The sub will start making the popping noise. What is strange is that I have used the REW software from this site and the curve is pretty close to flat from 20hz to about 40hz or so. I have set the subsonic to 25hz as the 20hz gives it even more of a boost which makes it pop. The sub sounds great else where and is in a small-medium bedroom. I have used the REW software and can play 15hz notes on up and the sub sounds fine and does not make any noise's so the sub seems fine. During movies tho it is more of a quick 30-35 hz punch. I'm going to be building a new box with the 15inch passive in the 2.2 size box but wanted to see if there is a reason for this issue i have been having.

Thanks,