F20?

If you are going to do through the trouble to hide the sub, i'd just go IB- it'll be easier and will probably have more output depending on what driver/amp combo you use.

Sure, you can, if it will physically fit in the space you have. Others have done even bigger horns under their house.

As far as performance??? I know how an F-20 measures groundplane, I have no idea what your room will do to that. Before you go through the trouble, it would be a good idea to make sure that the opening is in a good spot in the room. With subs, just like real estate, it is all about location, location, location.

In most living spaces, rooms have more effect below ~~300hz (increasingly more so as you descend in frequency) on the response of woofers and subwoofers than the actual drivers do. "Behind the couch" doesn't give us enough information to be very helpful - other than to tell you it's a quasi near field position. Like Mike said... "Location, location, Location!"

Realistically you should qualify the room. There are freeware programs available that allow you to measure your room acoustics which are more capable than what we had for professional gear less than a decade ago.. (REW comes to mind).

Your best bet? Take a small woofer - place it in your preferred listening location -then measure the response at all possible subwoofer locations. If the one behind the couch works - then use it!

+1 to what kicker and lilmike said.

if that baffle location causes big nulls at your listening position, it won't matter how good the sub is, the overall experience will be poor.