I also have a big space. I also have mains that do a pretty good job on their own but sometimes could use a little more oomph.
One suggestion would be to add a 15" sub...sealed if its for music only. Then run your mains full range and bring in your sub around 50hz give or take...and then just live with it for a month or so.
There are a lot of sub options. Here is one that I can say really sounds good and gives you 60 days to decide if you like it...but as I said, there are many options that will sound great and get the job done....but with a big space, err on the side of a big sub no matter who you buy it from.
One more thing, if the sub is really going to be crossed 60hz or below, remember that there isn't a heck of a lot of tone being created down there...its mostly grunts and groans and booms and ambience so don't get hung up on spending a ton of money for better tone....there isn't much down there. On the other hand, if you are going to cross at 100hz, that is a different story.
As for location, mine is 2' off to the side of the right speaker...but centering is more critical if the bass output on the mains is weak.