Doc,
I gather you are looking for a sub for music (not home theater)?
I make this distinction because in my personal experience, I like it when an explosion on HT gets my adrenaline pumping, but have found the setup that does this well is absolutely a horrible setup for music!
Where I use the same sub for both scenarios, I have bought and tuned my sub for music, but set it up for the sub to play ~8dB louder when watching a movie (I will bypass this setting if watching a movie where the music is important - Amadeus vs Avengers
).
I would recommend that you avoid the SVS subs. SVS has (IMHO) decided as part of their marketing to be a sub that calls attention to itself. IOW, if you go to Best Buy and listen to their sub, you will no doubt be gratified by the depth and power of their subs; however, for music, it is not about being impressed by the sub (which is good for sales) - it is about the sub blending with your mains in such a way as to make the mains sound better, not like you have mains with subs. That said, I suspect their sealed Ultra 13 is not a bad sub for music!
Personally, I am not happy with the way subs are evaluated. We need a repeatable standard for evaluating them; however the "anechoic chamber" approach (usually approximated by measuring outdoors away from wall reflections) is not good. The problem is that as soon as the sub is placed in any normal residential room, you will get a substantial amount of room gain from the reflections and even more with corner loading. There is nothing inherently wrong with the "anechoic method", but rather
it is the idea that you want a FR which is flat down to 20Hz in an anechoic chamber that is ill-conceived! This will not sound good listening to music in your room after you add in room gain (which is often more than 10dB). The bass will be bloated and detail will be lost.
Here is an example of anechoic vs in-room charts as measured by Ed Mullen:
"Anechoic" (ground plane) measurement:
In-room measurement:
As expected, room gain helped considerably to improve deep extension, with the Polk holding flat to about 25 Hz, and then dropping to -10 dB at 20 Hz.
For the complete read:
http://hometheaterhifi.com/volume_12_2/polk-psw-505-subwoofer-4-2005.html
My statement would be that room gain added somewhere between 10 and 15dB to the output at 25Hz. Thus if you had an anechoically flat sub, you would have bloated bass in room.
Also, no one has yet mentioned the importance of sub placement and doing the "sub crawl". Because bass is so easily influenced by the room, you get standing waves in your room which can be nodes (exaggerated bass) or nulls (the bass gets cancelled). While it is still worth doing the sub crawl, I believe dual subs is the best way to reduce these effects.
As an example, I would call your attention to this article comparing a single 12" sub to dual 8" subs:
http://www.audioholics.com/subwoofer-setup/two-subs
Below is a graph at the primary listening position with everything run direct (no EQ) as it would be from your Yamaha:
The green is dual 8" subs and the purple is a single 12" sub. The dual sub FR is much smoother and totally eliminates the nasty 10dB suck-out at ~48Hz!
Understand that we are more measuring the room acoustics of a single vs dual subs than the ability of the specific subs.
So, my advice to you is:
Option 1 - get a pair of SUB-1200 shipped for $300! These are actually pretty tight subs. I can say from personal experience that this ported sub is more tight and articulate than my sealed SVS SB12plus. You can sell this sub on Craigslist all day for $100 each, so not much to lose if you want to try them out or use them as a stepping stone/learning platform until you decide on your final solution. But it won't surprise me if you decide to keep them. They do a great job. They can be this inexpensive because they are keeping it simple and not attempting to be anechoically flat down to 20Hz.
Here is an "Anechoic" measurement of the SUB-1200 (red):
Option 2 - Dual RSL 10S's for $800. (note that the size difference between these and the SUB-1200 is not as much as you would guess) You seem enamored to the RSL Speedwoofer 10S. While I have not heard them, everything I have read about RSL indicates that they are well designed, and the FR looks like it would be good once room gain is added, and I think they are a reasonable option:
http://www.soundandvision.com/content/rsl-speakers-speedwoofer-10s-subwoofer-review#2qHTUUD4oT53FGuj.97
HSU
I believe the ULS-15 is a very good sub, but a pair of them is well beyond your budget and having dual subs is more beneficial than a better single sub (for music, anyway). Hsu is clever in that he offers tuning such that EQ1 provides flat response down to ~20Hz, which is a popular (and, in my mind ill-concieved) benchmark and EQ2 which fits my model for good in-room music sound!
EQ1:
EQ2:
