"11-30-14: Bob_reynolds
Since music is the priority, a sealed cabinet would more likely be a better choice. And there is no reason to get a power amp. You do not want to drive your main speakers more -- this only creates more distortion."
No offence, but that's silly. There's more to audio than reading a spec sheet. And in this case, you're guessing in order to fill in the blanks. In the real world, you're far more likely to get a speaker to distort by under powering it. The OP has a big room and his current amp, while of good quality, is not all that powerful. If a speaker has to be driven harder than normal because the room is big, its far safer to do it with a more powerful amp. You'll get less distortion, not more. (Unless, of course, you're foolish enough to push the system so hard, it starts to break up. After reading the OP's comments, I really don't think that will be a factor here.)
Take into consideration also, that driving the OP's current speakers to their potential, will make it much easier to integrate a sub. The integration works both ways. Just as you can push an amp and speakers into distortion, the same can happen with a subwoofer and the amp that powers it. In fact, you're much more likely to have a distortion problem with the sub, as opposed to the main speakers.