2-channel listening with a sub-woofer

I use a sub with my V R4 JR's... an old ten inch Velodyne. I drive it off the preamp second set of outputs. It's old, not a seemless integration but it does add some weight to things and I like it... but like some military bases it's slated for closing... but yep, I'll probably always use a sub in two ch. Of course that is until I can swing speakers that do not require one... and maybe even then... just something about the authority it lends to the music.

I recently added a Velodyne DD-15 to my Merlin VSM-MX 2-CH system. First I spoke with Kurt at Vel's Phoenix support, who is quite helpful. He said that the larger 15" & 18" models are faster and cleaner than the 10" & 12" models due to the fact that the amp has to work harder with the smaller drivers. I think the variable servo control is also important to getting tight bass for 2-CH applications. The lower the crossover frequency, the less obtrusive the crossover, and the more omnidirectional the bass. With a low crossover point it's less likely you'll need 2 subs (unless the room is very large.)

I am using the Velodyne DD12 with Revel Performa F-32's and I just don't listen any other way now. It took some time to get it dialed in at the proper setting and volume but it is as if they were designed to be together.

As usual, small diameter drivers are said to be "faster".
Frankly I have never heard a "fast" 16 foot organ pipe. I think that what we really mean is that the cone is under tight control of the power amp, and this depends on the power of the driver "motor" and the mass of the cone rather than the diameter of the cone. If you have an 8 inch driver and a 10 inch driver from the same manufacturer's line, and they both have identical voice coils and magnets it is reasonable to assume that the 8 inch will give "tighter" sound. But if you compare a typical 12 inch driver with a 2" voice coil against a 15 inch driver with a 4" voice coil and twenty-odd pounds of magnet one cannot make assumptions. Throw in servo control, and all bets are off.

I use Martin Logan CLS 11Z and a depth sub. Three 8 inch drivers made by the same company that made the main speakers in a non HT set up in a very large room.
Try to stick to the same company for your mains and sub.Saves a lot of trial and error and mismatching, but not 100% fool proof. That's where your ears come in.