I think that I am trying to make it sound more like if I had towers vs bookshelfs!
Nothing wrong with bookshelf/standmount speakers. My private rig (not in the greatroom) is using standmounts. And killer subs!
I'm not really interested in trying to get you to buy something different, but your satisfaction, or lack there of was coming through in your messages.
If you really wanted to keep with the polks, you could add 3 or 5 more... it will get louder, and you will experience more even bass throughout your listening area, but those little guys are not up to the task of filling your room.
If you want lower extension, more punch, and greater output, my advice would be new subs. 1 pb2000 or VTF2 that I mentioned above would be a great improvement over 2 of those polks, especially considering the volume of room you are talking about. 2 of either of those subs and you might just find bass contentment.
EDIT: but if sheer joy of bass, bass enlightenment, vibrational nirvana is your goal... well, we can point you toward the mountain top, too!