Adding a sub. - can i start with one?

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."

snapsc

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 . . .

Totally disagree. You just described poor bass management / reproduction.

I hear tone, timbre and PRaT - from classical to jazz to rock & definitely electronica.

daveinpa- as illustrated by hilde45's response to my comment  on JUST ONE sub. 

Yes,no and....the poster who is compelled to go beyond with their "absoluteness."

Sigh

It is a simple test.  Turn off your mains.  Turn on your sub and set the crossover at 50hz and then play a number of songs...not much tone.  Walk around the room...still not much tone.  I've done it with four different brands of subs and it always ends up the same.

But, when played with the mains, the additional grunt and the additional ambience provides the illusion of a change in tone...but when you listen to the sub alone, crossed low, not much tone there.

That has been my experience...no doubt, mileage may vary.