Eh hem!...Subwoofers... What do ya know?

(1) One sub is not enough, for reasons given by @millercarbon. (2) Subs can be used to extend LF, but the most important use in many systems will be to fill nulls unavoidably caused by main speaker placement and room geometry. (3) It’s easier to get them right if you have measurement capability. (4) I prefer to have a high-pass filter on the mains and a low-pass filter on the subs. (5) If you know when the subs kick in, you've done something wrong.

@millercarbon - do you run your array in mono? Just curious.

@millercarbon - Thanks for the reply. I have had the same experience, that a well-integrated set of subs *seems* to image with the rest of the range. I hear the upright bass coming from in front of me, even though the two woofers are behind me.

I've been intrigued by the DBA since REG's review in TAS. But given the lack of space in my room & the great results from 2 mains + subs (which it took years to place), I haven't tried it yet. Maybe someday.