High quality subwoofer - which one?

I’ve owned my C4’s for about six months now, and they are excellent. But, you can’t ask a speaker to things it isn’t big enough to do. What that is, is to go FLAT to 20 Hz. The Vandersteen Quatro’s had built-in 250 watt class B powered subwoofers that are to die for once you hear them. The blemish on the Quatros, and it is a small one, was the slightly veiled lower midrange. The C4’s are absolutely seamless everywhere they go. The lowest bass can be fixed and most would probably say it’s not even necessary...until you hear a real sub system that is. Once you hear the effortlessly smooth low-end of a dedicated sub woofer you just can’t forget about it.

I started looking at subwoofers, and they are expensive, but relative to what this hobby’s cost(s) can be on some item(s), they are affordable. Well, ONE is affordable, anyway. I have three models that seem to all be nice products, and was curious what the feet on the street have used and your likes / dislikes. One model that I have not included needs an external hi-pass filter and I may be dismissing this model out of hand…and shouldn’t be and if you use this model, convince me.

I have a pretty large room (16’ wide x 40’ long x 8.5’ high (ceiling is split at 7.5’ and 9’)) so a bigger sub may be needed. Right now, I’m thinking ONE sub crossed-over pretty low. The C4’s do go pretty deep, but I probably agree that two would be “best” but expensive and the narrower room might make double placement hard. One is a snap, between the speakers away from the wall or near the right corner. I do not play “loud” (80-85 dB SPL average based on my radio shack SPL meter) but still like the smooth deep bass of a sub. Crazy loud isn’t the issue, but the sound quality is.

The choices are mind boggling in cost (JL Audio in mono or stereo) or mono (VELODYNE DD15+) verses stereo (SVS PB13 Ultra) set-ups at the same price. I have three options below to illustrate this.

The set-up that seems too expensive but outrageously good;
JL Audio Fathom f113 - $3600.00 / 13” / 2500 watts peak class D amplifier / sealed box.
But the sub needs a high-pass Cross-over like a Bryston 10B-STD @ $3,500.00

This is expensive compared to the SVS, but is it better than TWO SVS units?
Velodyne DD15 Plus - $4,000.00 / 15.0” / 1250 watt continuous class D amplifier / sealed box.

This seems like a good price but is the product good enough even if the price is? Use TWO of these in place of one DD15 Plus?
SVS PB13-Ultra - $2,000.00 / 13.5” / 1000 watts continuous class D / ported box.

So what have we done with subwoofers and why? This got real complicated real fast. Ported, vented, stereo at the same price as a mono sub ETC. The odds of auditioning these monsters are about zero as you hear the room. My room seems to be pretty good down low based on my Quatro set-up with powered subs. If a decision isn’t easy, I’m missing something…a lot of something’s it turns out.