Subwoofer while running bookshelves full-range

Go with the SVS,it gives you the three options to play with. BTW I used the SB-1000 with Harbeth P3's running full range with great results. And the icing on the cake is that you can return it if it doesn't work out!

I run mine from the speaker taps in parallel. I do the same thing in a separate HT system with center and R/L channels plus LFE (3 subs). Many people struggle with gain and phase to blend them. X-over is different altogether. Some crossover the sub below the lowest frequency of your x-12s. Some open the sub x-over all the way and use lower gain and phase (especially if it's adjustable) adjustments. Sealed subs aren't mandatory for music, my ported AAD sub is plenty fast enough to keep up with my Fostex full-range drivers. Many different quality subs can work. I agree with the SVS recommendation. One last thing regarding placement. Don't just assume a corner is best based on others opinions. I find having the sub a little in front of the x-12s plane and between them sounds best in my 17D x 35W room.

Throwing this out there just for some opinions: In a very "hard" room full of compromises, 2500cft, at moderate volumes, cutting off at 60Hz, does sub quality even matter as much, between "acceptable" and "awesome"? Most of what I'm reading is about high SPLs justifying the price differences, beyond whether it's tight/accurate.

Technical/factual question: Pipe organs not withstanding, is there any practical diff for music between -3dB points of 27 and 24 Hz? One wonders where the -0dB points are, and where real usable sound really ends at the lower end, both on my Klipsch RW-8 and the SVS...

Scratch one of my questions, since I belatedly realized that the rca-in/rca-out route for cutting off the speakers at 80Hz wouldn't work in my case, because. My DAC doesn't have variable volume output, so the two volume controls (sub and amp) would be totally disconnected.

I'm likely to try the SVS to see if it's a noticeable improvement over the Klipsch, and if I can belnd it as well. However, as a time-sensitive thing, I can currently get an insanely small 8" discontinued Pinnacle for cheap and has speaker-level outputs (the only unit I found that still had them): it would be an upgrade, but not as much as the SVS, plainly, and it runs hot. But it would allow me to do an 80Hz crossover, limiting the X-12s. I guess one thing I'm trying to get rid of is a slight reflection in the 60Hz range from the X-12 reflex ports off the walls, which I can't do anything more about (compromises whatever I do). The Pinnacle can't do much SPL, but probably is enough for my listening level, it's smaller, and rated 27Hz -3dB. So I think it would work, too, and be $200 less than the SVS. But so far, I'm not hearing much cry for cutting off my mains at 80Hz as a must-do. Doing so was what made me start browsing subs, and then I found lots of love around the forums for the way I actually have mine connected, particularly with regards to making the sub sonically disappear by lowering the xover.
...sorry for the ramble. I fully realize the correct answer is "just get the SVS and listen for yourself." But your experiences are appreciated.

In case anyone with similar questions to mine Googles their way into this thread, I'll answer some of my own questions, based on obtaining an SVS SB-1000. Although I started integration of the new sub from scratch, my ideal blending ultimately arrived with the same crossover setting of just under 60Hz. Higher gain (1:00+ instead of 11:00), which seems to follow from a sealed versus ported design.

1) In a room full of acoustic compromises and listening strictly to music (90% rock or jazz), with a goal of hearing rather than feeling the bass at moderate volume, is a "good" subwoofer important, over a mediocre one?
As one way to investigate, I plugged in my old sub to my DAC directly, set the crossover to 40Hz (it's lowest setting--the SB-1000's is 50Hz), and played some tracks heavy with bass guitar. Although I read on a sound engineering site that bass guitar only extends to 60Hz and upright bass to 40, that seems wrong. I couldn't find any album without significant articulate substance below 40Hz. On Tony Levin's bass riff from "Don't Give Up," for instance, only the very highest notes rolled off vanishingly . My old sub was rated to 32Hz (-3dB), which gave virtually the same performance as the SB-1000 on "natural" bass (kick drum, bass guitar, upright bass), though the SB-1000 may have more definition even here on the very lowest notes, and has a superior tone--I want to call the SB-1000 Darth Vader's bass amp. But, for tracks with augmented bass (synthesizer, for example), there is obviously a plain difference between a 24Hz rating vs. 32Hz rating. "Welcome to the Machine" became a different experience with the SB-1000, there were also low-level room ambiences I didn't know were there on various tracks. Meanwhile, hip-hop,Erykah Badu's _Badduizm_, and Brian Eno's _Another Day On Earth_ obviously contain notes a 32Hz sub can't even find. If I weren't consciously searching for bass tracks, my normal round of rock and jazz is served 95% equally by both subwoofers, save for the fact that the very lowest tones are much juicier through the SB-1000. That juiciness, unfortunately, doesn't quite match the tonality of my Dynauido X12s at their bottom edges, which is noticeable once in a while in ways that didn't occur on my older sub. Current solution there is currently to turn the sub down more, which is a shame.
In all, I've been surprised at how much low-level content there is on a wide range of music, however, which can benefit at least to a small degree from lower extension.

2) Is high-passing the Dynaudio X12s at 80Hz preferable to running them full range with the sub crossing in at a lower frequency? I was able to experiment with this by connecting the SB-1000 to the DAC and then running another patch cable to the amp. After volume-matching and readjusting the crossover to 80Hz, I could switch between the highpassed setup versus full-range plus 60Hz sub crossover alternatives by turning some knobs and switching DAC outputs between RCA and XLR.
Unfortunately, other variables get in the way here. When running full-range, I normally use XLR inputs. These seem to sound better to me than an rca connection from the DAC, either because of cabling or XLRiness or delusion. But plainly, when adding the subwoofer's electronics into the chain, I at once liked the bass more (probably because 80Hz let in more juicy tone) but I liked everything else less. It lost some of the dimensionality, which may be a matter of noise floor, or may be a matter of losing the continuity in the X12's midrange driver.
What I can say is that "relieving" the X12s and my amp of the loest 20Hz didn't improve anything enough to overcome the barely-perceptible limitations of cabling or added electronics. Too bad I don't have the option of trying XLR from DAC to amp direct, plus amp-controlled variable crossover to the sub (e.g., a "normal" setup). But I love my class D integrated amp, so my current setup is quite a good balance of compromises. Nor do I regret the new sub, particularly since it sits directly below the shelf unit that holds my mini-components and so occupies effectively 0 space.