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.

To OPTIMALLY separate the (low bass) subwoofer passband from the (mid-bass) main woofer passband, it's necessary to use a full 4th order (24dB/octave) active Linkwitz-Riley crossover. A simple (generally passive) 2nd order (12dB/octave) crossover is simply not sufficient; the overlap is excessive.

Is a 12dB/octave crossover better than nothing? Well, probably; but why compromise unless there's a cost-related cause.

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.