gentle dudes.... as a convert to single, full-range, high-efficiency, crossoverless speakers for about 2 years now... let me share another revelation with you all, whom are looking for a way to add bass weight to your system.  RandyinCaintuck on this forum... enlightened me.... Hawthorne Audio Augie's - open baffle bass drivers.... Yes... "open baffle" drivers can and do produce bass - best I've ever heard.  I have two Velo digital drive subs, and I prefer the Hawthorne Augie's.  If you have not heard boxless bass, completely uncolored by any kind of enclosure resonances... you haven't heard HQ bass yet.  And to top it off... the Augie's are inexpensive, don't need anything but a piece of plywood for a mounting baffle, and you can get any inexpensive SMPS, class D plate amp to drive them... Or use that old class A/B sand amp you have sitting in the closet, that you never got rid of, after converting to glowing glass goodness....

DBC.... I could write you a novel about how I got to where I am.  But.... to summarize.  I have had the HSU ULS-15 in my room... great sub.  If they would have had a remote control I would have had them instead of the Velo's.  I wanted remote control of the volume for reasons I will elaborate on below.
I have seen and heard the HSU mid bass modules in HSU's room at RMAF... I considered them at one time because what most people don't realize is that they are lacking/desiring mid-bass weight not "sub" bass. I think this is a Dick Olsher, or somebody like thats philosophy.  Because our hearing sensitivity to bass decreases with volume, (Fletcher-Munson curve) most of us, especially when listening at lower volumes, don't think we have enough bass.  Or that the "balance" of the sound is off, too lean, not enough bass weight, so we think we need a sub.  What we really need is a good, transparent "loudness" control - Fletcher-Munson again.  The Velo's provided me with remote control of the bass volume and the low pass xover point and some other param's.  So, when the overall music volume was low, turn up the bass to get the "weight" that makes the music sound "right".  

Augie's... xover point... typically 60ish hz. With my full rangers, which BTW are Audio Nirvana's.  My room is far from typical.  My man cave is truly a "cave".... it's the entire lower level of our house, a 2400 sq.ft. walkout basement.. timber walls on 3 sides, concrete on one, concrete floor, 9 ft. ceilings.  There are walls, corner, etc. but basically the whole area is open.  It's the deadest room I've ever had.  Too dead to be honest, no detectable slap-echo at all.
I first sought out some "bass units" before I ever got my DIY full-rangers completed.  Everything I ever read, or have read to this date... told me that my full-rangers would have NO bass.  Well folks, I'm living, breathing proof, this is simply not so.  I put my Audio Nirvana 12 inch Alnico's into 5.6 cu.ft. reflex boxes, ported as recommended by Dave Dicks(Audio Nirvana owner)... and when I first fired them up, I thought OMG, how am I going to tone down the bass.  They were way to "heavy" sounding.  Good thing however, they quickly livened up and too much bass was no longer an issue.  I have been using these speaks most of the time now, for going on 2 years, despite owning 6 other pairs of speakers - these are my favorite when paired with my Torii Mk.3/Jupiters.
So... now I had these Hawthorne Augie's already purchased - what to do?  I built a pair of custom Hawthorne Trio's.  Two 15 inch Augie drivers per open baffle speaker panel - Four 15's total.  I have never, ever, at any price heard this quality of bass in any home stereo system.  And believe me, "quantity" is not an issue.  What is most intoxicating, is that they produce "tactile" bass even at low volumes... bass you can feel and that produces that sense of "pressure" that you don't really "hear" and they do it at low volumes.  Could not live without it after having it.
I have the Hawthorne std. coaxial units also for the Trio's, but am not overly enthusiastic about them.  But they can be disconnected, and I can use just the Augie's as additional bass units for my Audio Nirvana's in a box.  
Recently I acquired some 15" Audio Nirvana full-rangers to replace the hawthorne coaxial units... and then I will have a full range open baffle pair of speakers to play with.
DIY is not much of an issue with the Hawthorne Augie's... you can buy the baffle kit from Hawthorne to mount the Augie's in.. no cutting required, just screw everything together and get yourself a plate-amp... and you're in business.

DBC....

absolutely I've tried line-level & speaker-level signals to drive subs.  I, like you, have found the speaker-level method to provide a more synergistic, better integrated, more seamless bass from the sub.  The one exception to this that I currently have, is in our HT, where I'm driving the sub with line-level from the LFE of a pre/pro by necessity.  I was never satisfied with the quality of the bass or it's integration in the HT room.... until we had a reorg of that room and two things changed:
1) I was able to place the sub at the front of the room in the same plane with the front L/R/C speakers, and
2) I went to a sealed sub.  Sealed subs are just plain ole easier to integrate in my experience.

I'm sure the more advance pre/pro I got about the same time with room-correction software built-in helped too.

I've tried line-level in the 2-ch man-cave rig, but speaker-level just sounds better to my ears... most of the time, up until recently - read on.

Remember that part about "remote control" of the bass... to compensate for the loudness effect at low volumes. Or even recording to recording.  Well... a plate amp has volume controls.  But that would require me to get out of my chair.  I run my full-rangers "full range" with as pure a signal path as I can achieve.  But the Augie's have to be low passed.  I use an external electronic crossover for this.  Since I got tubular a few years ago, my sand amps were sitting around collecting dust.  I use a spare Class D Audio sds-470c to drive the four Augie's.  My crossover has input attenuators and output gain controls and the 470c has input attenuator's also.  But again, I would have to get out of my chair to "tweak" the bass.  Well... I just happened to have a really transparent SS preamp sitting around unused, also from my previous SS days.  Guess what it's being used for now?
Obviously this is all taking place with line-level signals.  But for whatever reason, with these OB bass drivers and perhaps other factors, the bass integration is completely seamless.  In fact it's so well integrated most people can't tell which source the sound is emanating from.  I get a kick out of watching friends, when first exposed to my current system, try to "walk to" the source of the sound.  They typically go toward the Trio's first, as they are the largest and most visible at around 5ft tall and 22" wide.  And because of the quality and quantity of bass, they are assuming "big".  But after they get in front of a Trio and run their head up and down the baffle and can't hear the mids and highs, they then gravitate to the boxed full rangers sitting to the insides of the Trio's.  Again, they tend to be a little baffled, because the bass sounds like its coming from them now.  It's about this time, when I get the puzzled looks and the questions start.  And when you get to the normal listening distance, which is about 10-11 feet, none of the speakers appear to be the source of the sound.  Completely holographic, wall to wall sound that all seems to start about 5-6 ft behind the speakers.  Which, ironically or not, 5-6ft is about how far from the front wall the speakers are positioned.  I need to pull them out from the front wall even further and see what that does... so many things to try and experiment with - so little time!!!!  Isn't this a great hobby?  I have never been happier with my system since I found Decware, full-range crossoverless speakers, and now OB bass.  I'm just not sure how much better it can get at this point.