I use two subs with my SC's, both are NHT SW2's, which have outboard amps. I have the subs on spiked amp stands, with cushioned auto wheel rings under the subs. How I came to that configuration is a long story, but they work very well. The only way you are going to know if two are better then one is to try them together. Purchase where you can get a return accommodation with full refund within 30 days. If one works, take the other back.
I had these speakers in an open living room/dining room/hallway like yours, and the REL Storm III I had worked superbly. Like REL recommends, I had the crossover way down and I never had the gain up past 12 o'clock. This setup worked perfectly for me. I am not sure what the current day equivalent to the Storm III is, but someone is selling a used Strata III (smaller sub but still probably would work in your room) for cheap on this site. Also you can get a T-5 or T-7 from Music Direct or Audio Advisor, both for under $1000. I would definitely investigate those. And no, you will not need two. I am speaking about 2-channel audio listening, not home theater and LFE.
As SC53 noted, a well implemented subwoofer usually does not need to be set at high gain and/or high crossover frequency. This is particularly true when music, and not home theater special effects, is a priority.
Multiple subwoofers are used to get a more even bass response, it is NOT generally utilized to get more bass. This issue was discussed extensively in this forum a few days ago; see:
The REL "high level" connection via the Neutrik Speakon cable connects directly to the speaker terminals of your power amp for the most balanced implementation possible of the subwoofer. The Neutrik cable has three strands, yellow, red and black; the yellow and red connect to the positive terminals of your amp, the black is a ground that connects to a negative terminal. That's all you have to do before you start listening and tuning the dials on the REL. By all means plan to use this connection, not the "low level" RCA-type connections to the REL. It will come with a detailed manual explaining all this.
I use a REL Strata III with ProAc D30R.. My room is 15x 25 works great have the crossover at 34hz.. I've been considering a upgrade but it's down on the list after my PS Audio DirectStream Dac upgrade and my Auralic Aries..
Sc53 is the Neutrik made by REL or is it separate?
Erik, I wish I coujld afford a D30R right now. I've spent some time listening to the D40R, it was magical. Has to be in my future though.
Bob, yes I am worried about that stress. While sending some bass energy to the sub will help, still it's a big space and I do listen loud. My RM9 will easily overwhelm the One SCs. I guess I'll just have to find out. If I end up selling the ProAcs the sub will be useful anyway. I wonder if it would be useful to set the high pass at a higher frequency? The woofers in the One SC are small.
Velodyne makes the SMS-1. Don't know if it has a parametric or if it differs from Hsu's network. What is the advantage of a built in high pass vs. the REL with an external?
Jim, yes REL provides the Neutrik cable, although you can also buy after-market upgrades too. I had one made by somebody here on Audiogon some years ago, now I see that Kimber makes one specifically for REL subs, I think Signal Cable has one too. Try the standard one first, and follow the tuning procedure in the REL manual. Setting the crossover LOW (perhaps counter-intuitive with such small monitor) works better 90% of the time, but your room and ears and music will be the guide.
If you are concerned about overdriving the main speakers, you should go with a fully active crossover/subwoofer system that actually cuts off the main speakers below the crossover point. With such a setup, the best sound will most likely be with the crossover set at a quite low frequency, as others have mentioned above. I almost always find that for music, a low crossover point sounds better.
I haven't listened to that many subwoofers, but, of those I heard, I liked the Martin Logan subwoofers that I heard coupled to quite small Totem monitors.
While the REL does not cut off the main speakers in the low frequency, I have been impressed with how easily REL subwoofers can be integrated into a system.
Bob, so I'm on a hunt for a crossover. I knew about the SMS but not it's cutoff range. Makes me think I may have to choose between having a too-low cutoff or going full range on the ProAcs. I wonder if there is also the consideeration of how high the subs work well, if I'm trying for a high cutoff.
Any ideas on where to find a crossover with high enough range? Perhaps I could have one built?
Rhljazz, good memory! I do go to Phoenix HiFi, but live a little ways off in Las Cruces, NM. I have talked about going to Phoenix HiFi in this forum though, so good eye. They are nice guys with lots of lovely speakers and I always leave their acutely aware of my earlier career choices :-).
Bob, this is from the SMS-1 manual. What do you think about being able to find a good crossover with the ProAcs and a sub?
www.velodyne.com 16 - Digital Drive SMS-1 Users Manual All six presets contain the following fields: Low Pass Crossover Frequency and Slope Adjust the upper limit of your subwoofers frequency response. Select a crossover setting, in increments of 1, between 15Hz and 199Hz and slope at 6, 12, 18, 24, 30, 36, 42 and 48 dB/octave.
Bob, thanks so much for the great info and pointers. I'll have some work to do to thoroughly check them out.
With the RM9 I felt I was pushing the One SCs to their conservatively safe limit in my 11x12 room. I think if I try to run them full range in my much bigger new space it will be too much for them, so I do need to find a satisfactory way to divert the lows to take advantage of 1/f power spectra. I also agree that 80 is too low due to the small woofers in the ProAcs. As well as that first order is not going to be decisive enough. I took a glance at Marchand and that looks promising. I run JL in my car, and the amp has a good tone, if dry compared to my tubes. Shouldn't matter on the sub.
I do wish it was not so difficult to find a very nice pair of Response 2.5s.
Sarah (Sc53) noticed I said I wished I could find some 2:5s. Well she saw a pair for sale here and let me know and I now own an absolutely flawless pair of 2.5s in oak, purchased from Frank in Phoenix. A much more satisfactory solution than trying to get the One SCs to handle the big area with their small woofers and a sub.Thanks again for all the comments everyone.