I think xover setting is an importance also.
Ive always really had bookshelves, I mean the polks IMO arent really tough floorstanders. With my ProCenter set at 60hz, the ProMonitors at 100hz (both recommended by Def Tech) and the sub at 120, it sounds damn good to me. I don't know much about floor vs bookshelf, but Id assume great floorstanders would beat great bookshelves any day.
Oh L2W, calling L2W!!!
Id feel my most important investment at this point would be a 2nd sub.