Adding a sub. - can i start with one?

Sure why not?

daveinpa-

As you can see, asking a seemingly simple question will never get a simple response back. Yes, no and maybe but...

A single sub properly setup will improve overall SQ-but it's subjective like EVERYTHING in audiophooland.

1:52 into the video. I experienced a couple of REL setups w/John Hunter. Even a single is quite convincing.

One advice I can give, based on a recent personal experience, is to not skimp on subwoofer quality -- buy the best you can afford. Case in point: I used to have a single T9/i sub and added a second T9i after a few months. It definitely made an improvement but not as pronounced as I was hoping for. I then had an opportunity to buy a REL S/510, so I sold both my T9i's to help fund the purchase. I can tell you that a single S/510 was better (in my system) than the pair of T9s. It brought out more punch, nuance, authority, and a level of finesse that the T9's simply couldn't match. I now have two S/510's and couldn't be happier. If I could have afforded it, I probably would have gone for 812's ... but I do want to retire at some point :)

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.

A single sub properly setup will improve overall SQ-but it’s subjective like EVERYTHING in audiophooland.

The improvement past one sub was not better in a "subjective"  sense. As if it was just my whimsy that I liked it better. No, it was better in an "objective" sense. It was better objectively in terms of (a) measurements (additional subs helped eliminate both peaks and troughs as measured) and from (b) an experienced listening standpoint (bass was clearer, more accurate, not boomy etc.)

Everything in audio may be particular (contingent on many factors) but it’s not all "subjective."