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

+1 on ML 800x for configurability
600x is $600, ported and might integrate better with ported mains

A sub MUST have 0 to 180 phase control, phase invert, 3 and 4th order slopes in addition to level and frequency.

The ML x series do and have the advantage of being adjustable from a smart phone. With the ARC microphone and Anthem Room Correction software, room faults can be somewhat mitigated.

IMO, you should roll the low end out of your mains. This effectively increases main amplifier power and makes integration with the sub easier.  Do NOT use the sub the sub internal XO. Depending on the room and sub, you may choose different frequencies. A single polypropylene, Teflon or polystyrene cap can suffice for a start. See http://www.ielogical.com/Audio/SubTerrBlues.php/

Learning to use something like REW [Room Eq Wizard http://www.roomeqwizard.com/] will pay huge dividends

Adequate low end room control is big, expensive and ugly. Judicious furniture placement and main/sub location can get you most of the way there. Just don't listen in a sparely furnished room with bare walls and floors.

I disagree that one can't hear phase in the bass. Having spent 1000s of hours in studios with time aligned monitors, getting the low end phase as good as it can be is essential. When it's not correct, kicks and snares are flabby, upright bass strolls and live orchestras have no focus.

A good sub properly integrated improves the listening experience out of all proportion to the frequency band.

The letter is "C" for correct.

@audiokinesis

Adjusting the continuous phase control provides much more than a slight amount of timing adjustment and my ears were quite capable of hearing when I had the correct phase setting. With such a range of adjustment it is possible to ruin the sound or optimize it with proper listening and technique.
Same for my car subwoofer- with time delay I was able to fine tune the proper delay relative to my main speakers and once you find the spot- car or home- the bass becomes effortless, deep, powerful and fully coherent through the range. The subwoofer then complements and works with your main speakers rather than fighting against them.  But again, the key technique was using beat pulses rather than pink noise.  

Thanks again, all, for an interesting weekend of reading and learning. I suppose one has to make a decision at some point, so will try to decide (after auditioning) between the SVS SB1000 and the JL Audio Dominion D110, which is unfortunately twice the price (but perhaps twice as nice).  I'm grateful for all of you for taking the time to weigh in on my question.
happy listening (and trails),
paul

You can lead a horse to water....