In floor designs are an intriguing idea, though to do them right a lot of planning and forethought is required.
First off the correct location needs to be selected. Keep in mind that the best sounding location for placement in the room (after proper testing) may not exactly be where you want to cut a hole in the floor. Nor may it be a location where the enclosure can be located beneath the floor as well. That being the case a proper eq such as a DCX 2496 would be of extreme benefit. in going this route regardless of whether or not proper placement could be achieved.
After location is sorted out then you have to consider how to properly design the system. A plenum is a good idea here, but then one must also remember that the vent or port cannot terminate into the plenum. It must terminate into the room itself. The primary reason is that the plenum will actually act as though it is a further extension of the vent/port. This will cause a misalignment of the actually tuning frequency of the enclosure and could drastically alter performance.
The next item to consider is that if going vented the woofer must also fire into the plenum chamber or directly into the room. The design is completely possible but takes a very creative design approach.
Then, isolation is also a very important item of consideration so that you do not cause rattling, vibration, etc. One will essentially want the woofer to be on a floating mounting system so that it is not hard mounted to any area of the surrounding structure.
An easy way around this is also the use of an infinite baffle in-floor sub system. Then one essentially just needs a plenum chamber to fire the woofers into. There would be no worry of the vent or additional enclosures to mount, etc.
Here is a great option for an IB sub system:
IB15
4 of these in a plenum chamber IB setup would provide stellar results. Combined with an EP4000 from Behringer with two of them on each channel and a DCX2496 you could be all set for less than $1K.
Hopefully my customer will allow me to complete the install at their home of a vented in-ceiling design I did so that I can get some pics to help you understand what I am describing here.
