New info so far;
REL and Vandersteen use a similar approach, but they BOTH lack any sort of in-room EQ, that is critical to performance. Great HP system and signal integration system on the Vandersteen, though.
-Vandersteen Quatro series speaker on up uses the same clever integration system as their subs, but add eleven band EQ. I wonder WHY this is not on their subs? It is essentially the same system, and they KNOW IT!
-REL is old school move it till it works and HUGE cabinets to get low-end response. Yes,it can sound good but you have no real room for two ported subs. And, they don't High Pass the signal. Somehow, REL seems behind a generation of room EQ. You may get it to sound good, but will you like where it/they are?
Too bad the Vandersteen doesn't have the eleven-band room EQ, I see this as a major goof that was developed in their upper speaker line. I wonder what gives there? So again, they seem behind, too.
The Paradigm SUB1 seems good till you realize it gives up the stereo option at any where near a reasonable price, especially with no real HP option that is affordable. The Sub1 is MONO and EXPENSIVE.
I'm not a proponent of running the C4's full range "over" a sub's response. You would never make any speaker this way so why splices one on top of another for the bass? It makes no sense to me other than HT / LFE doesn't care as much about accuracy.
That said, I've looked into STEREO placement and the time and phase information differs from channel to channel, so stereo (true right and left channel) subs is the way to go. It seems too many people neglect the differences between true right and left and just go for sound QTY. Two 10 inch subs offers more area(157 in SQR) than one 13.5 inch sub (143 in SQR)and faster transients, too. You lose a touch of REALLY deep bass, but should more than make up for it in stereo imaging, response and room smoothness. Dynamic range isn't a problem with two 10" subs.
The price is about the same for two smaller Velodyne DD10 Plus subs as one DD18 Plus. The stereo option is just too compelling to give up. And, the Velodyne brand will let you either go full range or High-pass at 100 or 80 Hz removing the C4's from low bass duties improving intermodulation distortion and imaging precision. To me, that is more important than 4 HZ on the bottom end or a few extra dB of headroom as I don't listen HT "loud".
So far it seems to be the DD-10+ or DD12+ x 2. You get;
Ability to high pass OR run full range.
Room EQ option (manual works great).
True stereo at a reasonable price.
Smaller size with a sealed box.
Good reputation for sound.
Great looks.
Talk me out of this.