Subwoofer Integration with Transmission Line Speakers

Or you could just plop four down, one on each wall, check the crossovers, tweak the levels, and be done with it.

avanti1960, here's my Fanboy spiel.

Velodyne's DD and DD Plus series includes a remote control, onboard Room Optimization Equalization, a calibrated microphone and stand, user computer software, CD player test tone CD, the option of using a laptop or TV allows visual monitoring while making adjustments between your main speakers and the subwoofer from 200Hz-15Hz. An extensive list of automatic and/or manual adjustment parameters between 100Hz-15Hz remotely from the listening position:

SETUP PRESETS (1-6)
LOW PASS XOVER FREQ
LOW PASS XOVER SLOPE
SUBSONIC FREQ
SUBSONIC SLOPE
PHASE (variable)
POLARITY
VOLUME (1-99)
CONTOUR FREQ
AUTO ON/OFF

Within these parameters is the ability to adjust Parametric Filter Frequency, Level, EQ, and Q, by dragging the cursor to match your main speakers using your ears and eyes to assist in your desired result. Its seems a daunting task at first but it soon becomes quite simple.
These adjustments, along with the other adjustments, can be saved in one of six presets. I have one set for flat room response another for inverted polarity and the remaining for different musical taste levels. By comparing subwoofer manuals setup procedures online you'll gain a better understanding of the detail of adjustability and low distortion Velodyne offers in their DD series.

The REAR PANEL includes connectivity for:

Left and Right XLR; INPUT, THRU OUTPUT, AND HIGH-PASS OUTPUT.
Left and Right RCA; THRU, OUTPUT, INPUT, LFE

LINE LEVEL CONTROL, SPEAKER LEVEL CONTROL

SPEAKER LEVEL INUT (binding posts)

S-VIDEO, RE-232 IN AND OUT, VIDEO OUTPUT, IR INPUT, 12V TRIGGER, ETHERNET. 

I used a single DD-18 which provided an indistinguishable level of integration and interactive flexibility at the listening position for years. I've since replaced it with two, more refined DD-12 Plus which are irregularity positioned in the room which reduced room nodes to the very deep corners. Yes, four drivers in the room would likely eliminate the nodes altogether but I find two units do a more than acceptable job of loading my room. Results will vary but I'm certain there is an acceptable result available to you. Have fun with it.

I have tried any ways to properly integrate my subs with my mains. After trying a deqx primate plus, I finally went with a DBX Driverack 360.  It is very versatile with delay settings for mains and both parametric, “Q” adjustment and of course graphic eq settings.  All drivers have independent gain controls. I use a laptop to control it and am very happy with the results.  (I’m pushing a set of Wilson Max II’s and a couple of JL 113’s in a room that measures 19 x 21 with 9’ ceilings)

There’s a Velodyne DD 12 Plus for $975 in CO. advertised here that goes way beyond EQ.