Two subwoofers for two channel music listening

^^^ REL recs are nonsense. Subs are 100% room and main dependent.

Right. No difference. Sub bass is mono. Line level or speaker level, same result. Running from one to he next is fine too. I've done all three. No difference. Use whichever is most convenient.

Unless you have mains that play well to 30Hz or lower, the subs are going to have significant output into the mid-bass which is easily localizable.

Mains with smallish woofers - [less than 10"] - will benefit from rolling the lows from the mains

Line level is the ONLY way to do it properly. Hi-Level inputs are a compromise to the amp, the mains and the subs.

See  ieLogical SubterraneanHomesickBlues for some useful information on sub integration.

I think only advantage REL offers with high-level is convenience factor.
I agree using line-level direct to subwoofer is probably a better approach.
Pre-amp (or using DAC direct line-level) to amp -> High-Level connection to Subwoofer -> Drop voltage to match internal line-level of subwoofer.
For sure line-level is more direct, but who knows if you can really hear any difference unless using long runs of RCA cable is having issues with RFI. Line-level using XLR will have advantage over RCA.

I got in touch with SVS and they suggested not mixing high level and line level. That we all can agree on:))

I will get a measurement mic in next couple of months or so and measure these things for my own learning and also for fun. I want to place the subs in their optimal location in the room - not necessarily symmetrically inside or outside of the main speakers. Nothing wrong with the symmetrical positioning though. Aesthetically they look really nice. Right now they are outside of the main speakers. 

I am also planning to do a DSP based crossover to distribute low frequency, say 50 - 60 Hz, to the dual subs. I used to have external 80 Hz high pass filter with my large bookshelf speakers and the dual sub crossover set appropriately. I really liked the sound. The imaging and soundstage were great. I had a pair of Maggies with the same set up. Now I have a pair of Thiel CS2.4 (and I am already looking for CS3.7 or 7.2 model ...... my never ending itch for upgrades - sigh) that go down to mid 30s, I guess. Adequate for most music I listen to. I still want the occasional 20 Hz bass if it is present in the music, as I like to feel that bass more than I hear it :)))  Heck at some point I will go for distributed bass array with 4 subwoofers! My room is a dedicated listening room, 17 X 24 ft in size. So I got space for that.

Before the mic and third party DSP ask SVS if one of their DSP/App equipped models could act as a master controller for your 1000 series. Remote control volume, customizable EQ presets, and the very useful adjustable Q = interactive low frequency.

I'm not sure if the SVS version is similar to the Velodyne Plus' Q which is a simple visual and audible drag and drop adjustment for individual bandwidth parametric filter frequency and level.
When used in conjunction with its discrete mutiband gain settings allow for a much higher crossover beginning with tone that closely matches the mains presentation from the crossover region on.



I'm in agreement with georgehifi and ieales descriptions. Comparisons with an owners REL $9K Studio III and two different amplifiers the low level inputs were our obvious preference. I wouldn't recommend the product.  

Richard Edmund Lord may have been the first to offer high level inputs intended as a connection of convenience for systems that lack low level outputs. Remarketing the same circuit for a different reason?
I would add that I've never encountered speaker level connectivity in any professional reinforcement or studio system.  

You want your subs to be the deepest, tightest and driest they can be, the last thing you want is an amp driving another amp, that’s just wrong and gives one note bass, that’s why line level drive the best for them.
The last thing you would want is to drive them from say a tube amps etc speaker output.
Probably the only amp driven output that could compete with line level, would come from an amp like a Krells or similar, that have absolutely no ? on bass performance.

Cheers George