This is a general response here, but please also clarify some more what you are thinking of doing and/or trying to accomplish with this. I'm not sure exactly what you mean here:
Suppose I added two subs ... would "chain" one to each stereo Submersive. Each of those could go where it would tame it's respective Submersive.
1) Two subs can provide a subjectively more expansive bass presentation. I honestly find this to correlate much more strongly with how two subwoofers may interact with a room more so than the exceedingly rare case of recorded stereo bass and reproduction of it. It can be very interesting to sit in your listening position and again at other points in the room while running a slow frequency sweep in the room. Depending on the acoustics of the room and the interaction of the subwoofer(s) with the room, you will often hear the output to subjectively "swim" around the room, changing with frequency. Sometimes rattling objects or large surfaces also impact this, and sometimes there are other effects behind this. 2) The benefit of 2 more subs would depend on many things, including placement and in-room response of the subwoofers. I suspect there might be other ways to better get at the end goal. Remember that for 95% of recordings, the subwoofers will behave as mono subwoofers if each is fed a Left & Right line signal. I often like to compare the subjective imaging and space of the main speakers with and without the subwoofers muted. Ideally the subwoofers should greatly enhance the scale, space and power of a concert drum and related sounds. If the subwoofer seems to be calling a little too much attention to itself, do experiment with the delay settings in the DEQX or your Meridian. If you add a small amount of delay to the subwoofer through distance setting (closer distance setting), direct delay (adding delay), or even through crossover (lower or steeper = more delay) you can shift the subjective interaction. Obviously you then have the fun of balancing adjustments related to imaging and localization vs. total frequency response.
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