Will a subwoofer add depth and clarity to my system, or just bass?

A couple of clarifications…

One of the major benefits to be derived from the addition of self-powered subs (use at least two, generally spaced wide, and towed-in from the front wall corners) is that they'll help you defeat room mode peaks/nulls. They're the next best thing to 4 inch thick broadband bass traps.

Another important benefit is that they'll allow you to take the heavy lifting off your main woofers/power amplifier. To do so, you'll need to install via an external active 4th order electronic crossover controller. Marchand's XM66 is a good solution.

Yes, proper integration is vital. You need to accurately phase-match the subs with your mains at the crossover frequency, and you need to assure that you've properly set the subs' input gain (at xover frequency) so that it's +3dB more than mains output. I have very recently posted a paper about how to do this (with instruments) on the Classical Candor website.

In quickly surveying this site it's evident that some commenters confuse phase-coherency with timing. Yes, it's vital that you synchronize the phase of your mains and subs (at xover frequency); however, that does NOT mean that wavefront time-of-arrival will be matched. An inherent time-of-arrival disparity will still remain because the related low-pass filtering entails group delay on the order of some 12-16 msec. in the crossover region, so subwoofer output will lag the main speaker by about one full wavelength (equiv. 13.5-18 ft.). Regardless, after the two signals are accurately phase-matched at the listener site little evidence of this timing offset will be apparent. A good self-powered subwoofer that offers a continuously variable phase angle control over the span of 0 to 280 degrees will provide sufficient additional delay to allow you to accurately phase-match with your main speakers' output. A 0-180 degree control is NOT adequate.

vtvmtodvm:

"One of the major benefits to be derived from the addition of self-powered subs (use at least two, generally spaced wide, and towed-in from the front wall corners) is that they’ll help you defeat room mode peaks/nulls. They’re the next best thing to 4 inch thick broadband bass traps."

Hello vtvmtodvm,

Your statement is phrased as if it were factual or at least something agreed upon by a consensus of acoustical experts or experienced home audio enthusiasts. Based on my knowledge and experience, I believe your statement is merely your personal opinion, is mainly demonstrably false and should be completely disregarded by anyone trying to attain good bass response in their rooms.
It has been proven that using a single sub in any given room can provide good bass response at a given single position in the room if properly positioned but the bass response will also be very poor throughout most of the remainder of the room due to the large number of bass room modes generated by a single sub (heard variously as spots of bass exaggeration, attenuation and even bass absence).
Adding a second and third properly positioned subs to the room begins to gradually reduce the number of bass room modes which not only increases bass quality and impact at the assigned sweet spot but also begins to improve bass response throughout the remainder of the room. Doctors of acoustical engineering, Earl Geddes and Floyd O’Toole, have discovered and verified that having 4 subs properly positioned in a given room (in what they call a distributed bass array system) will eliminate the majority of bass room modes and provides excellent bass response throughout the entire room, making any position in the room a bass sweet spot.
I use the Audio Kinesis Debra 4 sub distributed bass array system in my 23 x 16 x 8 foot room and can verify this system definitely provides state of the art bass response throughout my entire room.
I wanted to point out that placing subs in the corners of your room should definitely be avoided since this will produce exaggerated bass and only multiply the number of bass modes existing in any given room. I’ve actually never heard of any benefit gained by toeing in subs in corners.
     As to bass traps, mics, room correction and parametric equalization, none of this is required with a properly positioned 4 sub distributed bass array system.
I don’t currently have the time to monitor the rest of your post for other misinformation, I never advanced past your first paragraph, but will try to do so soon.

Tim

Tim—With respect to the use of subwoofers to reduce room-related resonances, hey, the more, the better! Of course, most listeners have constraints that prevent adding more than two subwoofers to a modest-sized listening room. Certainly two is my limit, and I suspect that two subs is as much as most people would tolerate.

Regarding location: Assuming a front wall position (that's the normal personal preference), and given that such subs should be OUTSIDE the main speakers, a 16 ft. wide listening room (such as mine, also yours) would then dictate that the subs be in the corners—or virtually so. In that event, some toe-in would likely be helpful. In practice, decor-dictated personal concerns will probably determine where the two subs must go, and that's generally in the corners too, as they're likely be the only open area.

The "distributed array" concept is worthy with one notable exception: It generally (?) recommends leaving the main speakers UNfiltered. While that can be beneficial with respect to reducing low frequency room mode response, it ignores the primary benefit implicit when using an active electronic crossover to separate the ultra-low (<100Hz) bass passband (subwoofers) from the low-&-mid bass passband (main speakers). That separation assures that the main speakers (and their power amplifier) no longer have to process any heavy low bass energy. Those low bass frequencies now get shunted to the self-powered subs. That's an important advantage, and it's way too vital to bypass.

@vtvmtodvm --

...
The "distributed array" concept is worthy with one notable exception: It generally (?) recommends leaving the main speakers UNfiltered. While that can be beneficial with respect to reducing low frequency room mode response, it ignores the primary benefit implicit when using an active electronic crossover to separate the ultra-low (<100Hz) bass passband (subwoofers) from the low-&-mid bass passband (main speakers). That separation assures that the main speakers (and their power amplifier) no longer have to process any heavy low bass energy. Those low bass frequencies now get shunted to the self-powered subs. That’s an important advantage, and it’s way too vital to bypass.

Glad to see this pointed out, because it’s an important benefit high-passing the mains as suggested. To quote:

Some audiophiles don’t want to introduce yet another active "thing" in their precious signal path, not realizing that adding the crossover is very much the lesser of two evils.

Actually adding a crossover is really a WIN-WIN situation:

WIN # 1) Since you are now NOT putting in 20 Hz - 80 Hz into the mains you are not using up the available LF cone movement with bass, so the LF cone in your mains is able to play its higher freqs (up to IT’S crossover point) much more cleanly. You get an apparent 6dB or more dynamic range. You can play your system LOUDER, and also with less compression distortion in the LF driver when you’re having that Saturday night dance party and you’re playing urban bass technopop at 110+ dB. Really.

WIN # 2) Since you are not putting bass into that same driver you are not Doppler modulating everything between 80 and 600, or whatever the next crossover point is. This means cleaner mids. By far.

WIN #3) You are not sucking current out of your main power amp at low frequencies, so there is more current reserve to play those highs louder...

WIN # 4) Since the cones aren’t moving as far at the low freqs the driver itself is not generating as much back EMF therefore the damping factor and all of its issues are greatly negated. And you don’t need to run silver plated cold water pipes to your mains as speaker wires because there is less current draw by the speakers.

WIN # 5) Freqs below 80 are now NOT causing transient intermodulation distortion with the higher freqs (and vice versa) in your power amp. Cleaner still.

http://www.soundoctor.com/whitepapers/subs.htm

You also, with your earlier post, pointed to the importance of the distinction between phase and timing, the latter aspect being my main issue with the multiple sub arrangement, which has coverage as its main goal (and not uniform time-of-arrivals). I’ll vouch for the importance of using only a limited degree, if any PEQ while having fairly uniform coverage via the much lauded, even preached-about-as-the-gospel mono-coupled 4-sub approach, but I find placing dual subs symmetrically to the mains, in true stereo, can hold a distinct sonic advantage, and this doesn’t rule out finding a placement that takes into consideration successful coverage with zero to limited PEQ; it may take more effort, but it’s (added) time I’ll gladly invest for the summed-advantage outcome.

For some reason stereo bass appears to be a controversial issue, with many claiming quite rigorously that the directionality of frequencies below some 80Hz goes out the window, and while I find there’s some merit to this (certainly insofar audibility goes) there are phasing differences nonetheless (when stereo information in the bass is found in the source material) that are clearly felt when running dual subs in stereo. This can come forth as a deliberate effect via especially electronica/techno music, and as an "illumination" of ambience in particular with classical music. Being audiophiles, many of whom I gather listens to classical music as well, why would you not take advantage of this?

LOTS of good information above. Read it, try the ideas and draw your own conclusions.
My experience with subs has been good. Depending on your speakers, a sub can add a more solid foundation for the rest of the music. Can't build a great house on sand!
I currently have Merlin TSMs and a REL Strata III which are all sealed cabinets. Pairing sealed subs and speakers is easier because of the sharp roll off sealed cabinets present. If I could find a well preserved version of that same sub I'll buy it.