Stereo Subwoofer Pairing

If this is a two channel system in a reasonably conventional room and your main speakers are making a soundstage at the listening position two subwoofers effectively placed should provide more than enough node free area at the listening position to begin with. 

I'd suggest using both your existing subwoofers with the .7s in a mono configuration as a starting point. Concentrate on finding their optimum room location which reduces the most room nodes.

If the REL cabling prevents your from locating your subs at most locations in your room you might consider running them using their low level inputs and very long economical interconnects from Blue Jeans or Monoprice cable to aid in their room location.

You should be able to mitigate your concern of what you term as overlap with proper crossover and gain. Frankly, I don't think this will be an issue especially if the subs are irregularly located in the room and running in mono.

When I compared a REL Studio III it worked best sourced from a low level signal processed through a Velodyne DD Optimization program. I am not a fan of REL subwoofers.  

  

Deep bass is not directional, and that has been shown by a lot of research. Yet, as some have argued here, it may sound as if these deep bass notes do indeed emanate from different speakers. The explanation is that the human brain uses the upper harmonics from the main speakers to locate the deep bass tones. You can do two simple tests. The first is to use only one subwoofer. Interestingly, and provided the sub has been integrated properly, even with one mono sub the deep bass will seem to come from the two main speakers. The second test is to use two stereo subs, but turn off the main speakers. You will not hear stereo, because the directional information from the main speakers is lost.

So you can happily connect two subs in mono and not lose anything. However, why not connect them in stereo anywany- is there a disadvantage to connecting two subs in stereo? Indeed there is, because two mono subs will produce the same signal, and will therefore be smoothed more completely by having two subs reproducing the same signal in different locations. This is why most experts and manufacturers suggest to connect two subs in mono.

Two subs are indeed better than one because they reduce room modes. Similarly four are even better, but this is a story of diminishing marginal returns.
In principle you have four ways to deal with room modes in a given room:
1 use dipole subs (Rythmik have a kit for a dipole sub). Dipoles excite far fewer room modes, so you address the problem at source. The disadvantages are that dipoles tend to be larger and require more power.
2 install bass traps. The down side is that they are big and ugly.
3 use multiple subs. Two are a big improvement, and four brings additional improvement.
4 use dsp room equalization like the Antimode 8033. Use this with one sub and the (big) improvement is quite localized. Use it with two and the improvement applies to a much larger area.

Ideally, you may want to use all four methods. In real life, a combination of two or three will already bring vast improvements. If you have a bit of space I would opt for two dipole subs combined with room eq. If the room is small and domestic tolerance limited, I would opt for two really small subs like the SVS SB1000 plus an Antimode. If you have space and the budget, by all means go for four.

Ideally subs should be the same model, but as long as they are not too different there should not be a problem. Two identical ones but with slightly different ages are not an issue at all. Speakers do indeed need to be broken in, but this does not normally take more than a few hours. With subs, you are unlikely to even notice (but wait for a few hours with the Antimode equalization).

willemj,

I'm new to subs and just about to integrate a couple in my system.

A commonly recommended crossover point for subs is 80Hz, which means the sub will also be still putting out sound above 80Hz.

That seems to be getting into the directional territory.  I've read numerous people say they can localize their subs when crossing over at that frequency.

80 Hz is common for HT. For music I prefer to use them below the main speaker's roll off frequency, i.e. rather lower, such as 35-50 Hz. That is also the advice from REL: set the low pass filter quite low, and the volume relatively high.

willemj

Deep bass is not directional, and that has been shown by a lot of research. Yet, as some have argued here, it may sound as if these deep bass notes do indeed emanate from different speakers. The explanation is that the human brain uses the upper harmonics from the main speakers to locate the deep bass tones.

It is absolutely true that harmonics can suggest that the bass you’re hearing is directional and often, when bass sounds directional, that’s the explanation. But it is not absolutely true that all bass is non-directional; differing phase between bass signals can create the impression of direction. That might be obvious if you really think about it, but it’s also been proven through research, including in work done on multichannel audio by Microsoft.