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