Depends on the sub, vents and enclosure. I have TC 1000's in downfire in my truck and they are practicaly touching the back of the box, but they are mounted at an angle so I would imagine there is still space for the pole vent, I can't mount them any farther forward or they will slap the carpet, I think they may already be slapping the carpet at high volumes //content.invisioncic.com/y282845/emoticons/rolleyes.gif.c1fef805e9d1464d377451cd5bc18bfb.gif . General rule is to leave 1/2 inch of space behind the sub if possible. I talked to AA about this with the assassin and they said it would be fine to have the pole vent restricted. On the TC subs the Voice Coil is kind of open so I would imagine I'm ok... Even with 6.5 inches you have alot of options... I'm not an SPL guy so I won't make any recommendations.
What are your price limits?