Do you use a Subwoofer when listening to stereo?

Subs make music seem more lifelike and natural, kind of like what tubes seem to do...it takes a little time to set ’em up correctly (I use a "swarm" of 2 RELs...2...albeit a small swarm) but once dialed in you’re likely in a much better place musically, and you don’t have to depend on main speakers of that .001% that could be considered actually "full range."

The Velodyne SMS-1 acoustic room corrector has a single XLR LFE input.  Is it OK to use a splitter to mix the LR outputs from the preamp for input to the SMS-1 -- assuming full-range preamp output into an LFE input is not a problem?

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At a future point will plan to have me built two 10"-loaded PicoWrecker tapped horn subs in addition to the pair of larger 15"-loaded MicroWrecker tapped horn siblings that I already use in my set-up (they share a similar tune), for a quad sub set-up/DBA. That should smooth out the response for minimal if any DSP-corrections with the bonus of even more headroom - not that the latter is strictly needed, to say the least. Subs are essential in most any stereo system, not just Home Theater ditto, and when properly implemented will add an extra dimension to the sonic experience.

No dbphd it is not!! You are likely to mono your entire system. You would have to take those outputs to another line stage then Mono that.

I wish to reiterate that just adding a subwoofer to a system without using a high pass filter on the main amplifiers is going to produce inferior results 99 times out of 100. This format exists only because if it did not the business would never be able to sell subwoofers. If crash and boom is all one is interested such is the theater crew then fine I suppose but for the accurate reproduction of bass? Not possible or perhaps highly unlikely. Listening to an acoustic bass solo such as by Ron Carter or Dave Holland you should be able to switch in an out of bypass without hearing a difference as long as a really low note is not struck. Then it will be obvious there is a subwoofer there. If there is a difference there is a problem even if you like it. If you like more bass you add it after everything is adjusted correctly.
Adjusting a subwoofer is not a straightforward process. There is choosing the right crossover slope and frequency, matching volume with the satellites and most importantly matching them in time. The bass coming from the subwoofers and the satellites has to appear sonically as if it is coming from the same instrument in time and space hopefully in phase.
Before digital bass management the only way we had of doing this was moving the subwoofers relative to the satellites which usually meant pulling them away from the walls were they performed best. One compromise after another. Does using a "swarm" system negate the need for this? Not at all. The signal from both the swarm and the satellites still has to reach your brain at the same time. Swarming smooths out the bass response within the room but this does not change the interaction with the satellites. The solution is the same regardless of the type of subwoofer system. You ping all the speakers individually and apply digital delays so that the sound of each gets to the listening position at the exact same time. Trying to do this empirically by moving speakers around? You might as well go looking for a needle in a haystack. 
I suppose many manage to reach a reasonable compromise using the tools they have but the results are far from perfect. For very critical listeners subwoofers can have a very bad reputation for this reason.
Adding bass is always fun until you realize it is making that bass drum sound like a gorilla barking. 
In short, you cannot get the very best out of subwoofers without a complete two way crossover and digital bass management. You may be able to come up with something that is reasonable but there are never any guarantees. This is the problem that lies behind the argument as to whether or not subwoofers are a good thing. For many they will not be.
For people who just want a lot of bass they will always be a good thing.
For people who want accurate low bass they can be quite the headache. That is a battle of your choice. If you get a Trinnov, Anthem or DEQX unit first you will be light years ahead on the learning curve.