Treatment
Working in a room with reflective surfaces such as you point out is difficult and I certainly understand your frustration. The problem may not necessarily be with your sub so much as getting the most out of it. So, placement is key. You may be tempted to move the sub away from the walls in a setup where the room is large. Doing this only lessons the impact of your sub, so, keep it close to the walls; keeping it close to a corner is even better.
The RSW-12 is a great sub, so it should provide you with a great amount of impact and it is well-matched to your other speakers. What you may want to consider is getting another RSW-12 sub and put it in the opposite corner of the room. Dolby's web-site has some information on exactly how to do this, but there are plenty of web-sites that include optimal setups for .1, .2, and .4 setups (i.e. 1, 2, and 4 subs on a system).
Also, whatever you can do to dampen reflection will help your sound immensely. It will allow you to hear and feel a more precise image of your bass. Reflecting surfaces such as tile and glass can drag a sound out and muddle its image as well as its origin and direction. If you can put a good thick curtain over your windows and try to have some kind of furniture over the tile, that will at least help clear up some of your bass problems. Also, keep in mind that you should calibrate your sound with the volume of your sub between at about 1/3 of its max. Most people crank up their sub volume when tuning which only reduces the signal coming from the receiver. That, in turn, provides you little flexibility (i.e. you can only reduce the sub's impact rather than both increase it and reduce it).
If you decide to go with 4 subs, I would invest in a seat-belt! ;D
Happy sub-hunting!
