Two subwoofers for two channel music listening

Still I will stick what REL is telling me over you. 

Like I said, go with what you want, maybe Rel has got a sus low level input on their subs, who knows, show us a circuit diagram for some clarity.

I have a couple of REL subs and love them.  The manufacturer is very specific about setup. The ideal setup (from their point of view) is the subs connected at high level (to the speakers .... they have slightly different ways to do this depending on the type of amp you have.  Connection to the preamp is verboten :) Their ideal position depended on the room you have outside of the main speakers is the option for my long room, second choice inside main speakers for other shapes.  I have the impression from this and others responses that there are a few ways to do the setup.  If I were you I would call SVS and ask them what in their experience is best for you particular situation.

^^^ 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.