Subwoofer for music only ?

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.

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room reflections are an entirely different matter

Got the SVS 3000 Micro in today and hooked it up. Sounds great. I placed it under the foot of my bed, Its located right in the middle of the main speakers about 3ft away. With the side firing subs it works out good and the app is easy to use. Now I just need to play with it a little.  Thanks for all the input.

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!

"Thus, my preference, IF subs, use a front facing stereo matched pair, located adjacent to your primary speakers. "

I would stay away from this type of installation, unless your value aesthetics over getting the best sound quality.

We position our main speakers to provide the best best overall sound, but their ultimate position does not guarantee that the woofers are in the optimum position for low end accuracy. Adding a pair of subwoofers will only address room deficiencies if they are placed with that in mind.

Keep in mind that whatever problems with the woofer positioning, will only be exacerbated by adding more low frequency energy to that same location.

For purposes of 2.0 stereo, you cannot do better than REL. GO TO THEIR WEBSITE AND THEY HAVE A TOOL THAT WILL HELP YOU SELECT THE PERFECT SUB FOR YOUR ROOM BASED ON YOUR ROOM SIZE AND SPEAKERS