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?

Check out this solution. I use it for both my subs and it definitely isolates the sub from the floor.

http://www.sweetwater.com/store/detail/GRAMMAII

Thanks for the info. Not sure, however, how this would affect 'loading'.

As an added reply, I should have noted that the Ultra-X12 being a downward firing sub with a downward firing ports, caused my concern. The question of 'loading' really applies to both the driver AND the ports.
My KEF LS50 main speakers are vented, with the port being on the enclosure rear surface. KEF makes a great fuss about placement of the speakers with respect to the enclosure to wall distance, and includes 2 port 'tuning' plug sizes to compensate for placement distances less than 500mm (about 20-inches). With that in mind, I am as concerned about BOTH the driver and port distance from the floor.
Thanks again for your interest.

I doubt that any lack of "tightness" is due to issues with how the sub is interacting with the floor. Even a very lightly built home has literally thousands of pounds of flooring, furniture and "stuff" weighing down the joists. If the home is on a slab there is even less chance of the sub really "moving" the floor. There is a higher probability that whatever "easy glide" pads the sub is sitting on are allowing the sub to "float". You can try putting a bag or two of sand http://www.amazon.com/Quikrete-1113-51-50Lb-Play-Sand/dp/B00005C6E3/ref=sr_1_2 on top of the sub to increase its mass in an effort to keep it from skittering around. If the sandbag(s) are slipped into a fancy pillow-case http://www.westelm.com/products/cloudy-abstract-pillow-cover-shockwave-t2609/?pkey=cpillow-covers you can even convince yourself / your spouse it is a "designer decorator enhancement"

Without knowing the total volume of the listening space and adjacent foyer (two story??) it is likely that there is simply a whole lot of space being excited by the single sub. It is not going out on a limb to suggest that there is some nasty mode coupling muddying the sound...

How best to fix this???

There is really weird relationship between how much POWER is necessary to get acceptable levels of bass and HOW MANY subs are used. Yep, MORE drivers == LESS power "leaked". Even harder to believe, using MORE subs will actually result in both a TIGHTER sound AND less need for "soundproofing" as there is less "mode coupling".

This might sound like some hocus-pocus dreamed up by the firms that make & sell subs, but the FACT is that even firms that make their money selling bass control / sound absorption products can confirm that ADDING SUBS IS BETTER than trying to increase "soundproofing" -- http://www.acousticsciences.com/art-noxon/buy-subwoofers-instead-soundproofing

In re-reading your original post I assumed you wanted to ensure that the sub did not move around and did not cause bass loading to your flooring. Once I re-read it I came away with the idea that you have an issue with the way the driver is loading the ports inside the sub. If you are having what some refer to as "port chuffing" due to air movement through the port, increasing the distance from the floor may affect your ability to hear it somewhat. The GRAMMAll is designed to both elevate and isolate from the floor. It is carpeted to ensure the chuffing is somewhat absorbed also.

Even if your house sits on a slab the sub has the capability to transmit significant energy to the floor and around the house at high volumes and low frequencies. If you want to do an experiment, play a 20-25 hz test signal through your sub and walk around your house to listen for items vibrating. I do this anytime I set up a new room to make sure everything is going to stay where I put it.

Sorry, but I have away for a while. I have tried changing the 'loading' of the Ultra-X12 driver and/or vent. The changes were definitely audible, but never sounded as 'tight' as the bass of my ancient Transtatic I (KEF B309 in a TL + KEF mid-bass in TL + ESL mid-treble , , the best overall sound of any loudspeaker I have ever owned). 'Chuffing' or other strange sounds not a problem, just the quality of the bass. I am beginning to think that realistic, really 'taut' bass can not be produced by a downward firing subwooferdue to the necessity of the driver to 'squeeze' the air out from under the sub + the vent tuning is effected by the sub-floor space.

Sub woofer placement is a key to getting good bass response. Have you ever done the sub crawl. Place your sub in your prime seat and slowly crawl around the room while playing a test signal of around 25 hz. The position where you get the best bass is where the ideal spot is for your sub.

Well, first do you know what this sub weighs? Hard to place on my 'listening chair'. Second, if I worry about floor loading, what would a plush cushioned chair smaller than the sub result in? Third, my 50+ years of audio has taught me that the sub needs to be near (preferably between) the main speakers; otherwise a multi-miked viola would have the bowing on the left, the finger work on the right and the bass (maybe) behind me. Most unrealistic.

Most sounds from a sub a low enough that humans can't determine direction very well. And yes, I do know what it weighs, since I have the LFM1-EX which was its predecessor and weighs 1 pound more at 67 pounds. I placed mine on a piece of plywood directly on my theater seating to do the sub crawl. My 50+ years of experience tells me that the sub needs to be placed in the room in the right location OR you will get muddy bass due to the room nodes interfering with the response. If you can't put it on your listening chair then substitute something that will hold it while you do the crawl. 4 cement blocks and a couple of two by fours will do nicely to support the sub. There is no way that the sub is challenging the floor loading in your house. It weighs significantly less than you do, unless you are a 12 year old girl, so it isn't a problem. I weigh 5 times what that sub weighs and I don't have a floor loading issue in my stick frame home.