Thiel 3.6 and 'grounding' / insulating

Congrat's on your house. Sometimes, with speakers that possess prodigious bass output like the 3.6's, it can be disadvantageous to postively couple the speakers to a more resonant suspended wood floor, especially if no carpet is laid, but it all depends on the particular situation. I would first experiment with setting them up the way you have been to see if there might be a problem at all. If there is, it is entirely possible that a combination of mass-loading the speaker base and distributing the load over more of the floor area as you propose could help out. The last resort for a severe problem would be damping/decoupling using some sort of compliant material underneath - such as carpet. (Or, if the floor in question is a ground level, you can look into ways to reinforce the floor from below - especially directly underneath the speaker positions - providing a more solid connection to the foundation.)

Just curious Pvanosta, what is your amplification? I will say in my experience with my 2.2's, speaker cable of a heavier guage does work best for the bass, but only in terms of tonality, quantity, dynamics, and extension - lighter guage cable will just shortchange the speakers and give a thinner, less developed sound. The bass being somewhat 'loose' is probably a function of the amplifier chosen, and the 3.6's with their larger bass driver and radiator, and lower impedance dip, are tougher in this regard than mine. Since you don't mention it, I assume you don't consider this to be a potential culprit though, but then again there are many 3.6's out there in the world which are not partnered with truly adequate amps (or cables).

OK, I can see why you didn't suspect the amp...
I did own an older Classe once (a Seventy, not the same generation or as high-power as yours) that never sounded properly balanced until I put a Synergistic AC cord on it, but that really transformed it, made it much more smooth and open. If you can, do you use an upgraded AC cord?