Infinity Perfect 12DVQ Sub problem!

I have Inifinity Perfect 12DVQ sub in the sealed 1 cu ft box bolted on underneath the stock sub hole in my GS430 on the rear deck, shooting through to the cabin, I wanted to try this setup for SQ bass instead of facing sub towards the trunk lid on the floor. It is dual voice coil sub. The sub is powered by JL Audio 500/1 amp. The shop I found was a well respected sub that partisipates in Nation SQ championship and won 5th place with their Audio S4 last year. That car sounds amazing, best loud SQ I ever heard. The whole car was done with top of the line Rockford stuff. So I went with them. I bought my own stuff, including wires on Internet and brought it to them to install. They charged me $1200, that included Dynamat on both sides of the rear deck, and all the doors underneath the custom MDF door enclosures they did, custom sub box and AudioControl LC6 unit, I left my stock radio in. The over all sound was good, the setup was very clean. The speakers I thought were too bright and tinny. I used Infinity Kappa 6.5" Comps for front and coax for back doors powered by JL Audio 300/4 amp. The sub is rated at 400watt RMS, but it breaks up and farts at loud voulmes, I mean I do get bass and it is tight but it breaks up and when I look at the sub moving while it is working, it looks like it moves to much and does not look tight playing but loose and woobly? I could not bring the car back to the shop because I moved to FL from NJ few days after install and took the car with me. I went to one shop they listened to it and said the sub is bad, the other shop said it might be wired wrong inside the box and want me to comeback next week for them to look at. It always sounded like that. The sub was brand new, bought on line. Any idea what to expect or what to check for? If it is the sub, they sell JL Audio there, should I go with 12w3 for replacement? Anyone has any ideas?

these are the pics of the install:

http://www.clublexus.com/gallery/showgallery.php?mcats=all&si=&what=allfields&name=dlomonosov&when=0&whenterm=&condition=and