I have had an Ultra-X12 subwoofer (used in the maximum extension mode) since its introduction, to use with my RR2150 receiver. Overall, I am very pleased with the sound quality when paired with my small KEF LS50 speakers, though it took a while to adjust the levels and X-Over frequencies ('Pink Noise' + a 1/3rd octave analyzer). However, I still have the feeling that the bass is not as 'tight' as it could be, possibly due to a driver/port 'loading' problem. As I have hardwood flooring, I do use the supplied 'discs', PLUS (at my wife's 'request') I added under each disc an 'Easy Glide' padded disc to make the subs easily movable for floor care/cleaning. Thus, the overall height from the floor to the subwoofer bottom is 2-inches, probably twice the distance than just using the spikes on deep, plush carpeting. Too high? My listening room size is about 26'L X 18'W X 10'H (and is connected directly to a 12' X 8' Foyer and side corridors). It would be easy to add a layer of carpet or (properly damped) wood under the sub driver and/or the one open port to correct the 'loading' (and prevent driver damage). Any thoughts or suggestions that might provide some guidance?