Subwoofer for music only ?

btw, square rooms can be problematic, perhaps others here can tell us how they solved square rooms.

and, for Video, Small Home Theater like mine, I use a single sub, just to get things like Jurassic Park Dinosaur Stomps, Black Hawk Down ,,,,, Room filling Impact, not directional unless you have a large Home Theater, then, a pair!

Bass get’s directional because of the overtones. A single sub prevents the Imaging directional Bass creates.

Thus, my preference, IF subs, use a front facing stereo matched pair, located adjacent to your primary speakers. When a pair, smaller subs can be successful, easier to fit.

Subwoofer Ports: I am not a fan, if so, front facing.

A major reason to use sub-woofers is so you can use smaller primary speakers WITHOUT ports in them, (which exist always to get more bass from drivers that cannot do it by themselves). Adding a stereo pair of Subs, then the primaries do not need ports! You might try blocking primary ports when you add subs (another reason to use two). Better to plan the combo of primaries with no ports and subs together initially than to add subs later.

My main speakers, 15" woofers, when designing the new cabinets, with the help of Electro-Voice Engineering (they were still in NYC then, and extremely helpful), we calculated tuned rear ports, to eke out a speck more from the 15". A youthful and wrong headed experiment I realized. In prior space, no wall behind them, the ports were a slight asset. Moved here. rear wall too close, I blocked the ports.

IF anyone buys speakers or subs with rear ports, I advise at least trying them blocked, make a decision.

I use this 3 lp set to evaluate bass, adjusting my speakers, evaluating cartridges, ...

The Bass, 3 LP set, Excellent Performances as well as Famous Bass Players

btw, a pair of self-powered subs, pre to sub/equalizer, bass to sub, upper bass/mid/highs ONLY back to amp is a very good method of trying tubes, because less power is needed from the amp, thus more affordable, lighter, less heat, increased location options.

And, the smaller drivers will sound cleaner because they are not trying to do what they really cannot do.

Stereo Bass, Bass with Imaging!

overtones, harmonics

overtones emanate from the primary note, thus they are from the driver that makes the primary/fundamental note, (not another driver).

doesn’t matter what your crossover is set at, whatever primary note a sub (or any driver) makes will have it’s own overtones. (from that driver, from that location)

Say sub makes a 30 hz ’wide wave’ room filling mono bass note. there will be overtones: 60hz; 90hz; 120hz .... that become directional, enhancing imaging.

By the overtones you perceive the location of that sub making the primary mono 30 hz, You/your brain deduces where the fundamental comes from.

that’s why you want a stereo pair of subs, front facing, adjacent to your primaries.

no ports, if so, only front facing, not letting sound from the rear ports muddying the primary, in any way, even very slightly.

there is a ratio of decreasing volume as it relates to the overtones, perhaps someone can clarify that.

.....................................

room reflections are an entirely different matter

glad OP’s sub sounds good, most of my yap is about larger spaces, not partially filled with a bed.

rear surround, Sub(s) for Video is a different matter.

earlier I wrote "Black Hawk Down ,,,,, Room filling Impact, not directional unless you have a large Home Theater, then, a pair!".

You don't necessarily want the lows from the front for video, IF the content wants the fundamental impact coming from the rear or side.

Black Hawk helicopters, Dinosaur: coming from behind, over there ... are a great way to demonstrate the advantage of 5.1 surround’s REAR speakers. And, where's the sub, i.e. where does the director want the fundamentals of that Dinosaur Stomp to come from?

I don't know anything about large Home Theaters, 

I have only 1 sub in my small home theater, and it is adjacent to me, under the end table, facing forward. It takes some diddling with to get the crossover and volume 'right'. It, combines with the directionality produced by the rear surround speakers. My fronts have 12", so I only want 'big' stuff from my sub, otherwise, don't want much out of it. 

Many people set their rear speaker’s (and sub(s) volume too high. Normally, you should not be aware of them, but if you turn them off, the sound/image collapses to the front.

Happily existing without calling themselves out, until they perform their real purpose which is rear/side directionality and big impact.

Pseudo Surround can be worse than front 2 channel Stereo. i.e. watching "The Voice" singing contest. Donna and I often say to ourselves "they must be hearing things we are not" because that was not superior in any way. Change your AVR to Direct, or 2 Channel, it is surprising how much better it sounds. Try this for other content, often, if the original is not recorded in surround, it sounds far better in 2 channel.

When it’s good it’s good! A terrific sounding (and visuals) DVD, one of the very first:

James Taylor at the Beacon, 1998

We were there, it was an awesome concert and the DVD captures it!