Separate subs for music and HT/surround

db, were you able to get your subs repaired?   
   I think the SMS-1 has some memory presets that will allow you to EQ one unit for both stereo and HT. There are two manuals for your SMS-1 the Velodyne and the Outlaw. http://velodyneacoustics.com/product-manuals
   My DSP experience begins with the Digital Drive 18" and my current two DD-12 Plus. You should gain some insight by looking up both Velodyne's DD Plus User's Manual for multi sub connectivity pages 16-20 and the User Interface Manual. 
   You'll notice the subsequent additions to their software most notable the Frequency Response and Parameters Screen on page 10 of the Interface Manual. Following the Auto EQ program this one feature allows you to click on a frequency band diamond and manually make three adjustments siamotainously within that one band. 
   This adjustment allows you to closely duplicate the Q and filter the subs to closely match your main speakers presentation from 200Hz to within the crossover region. Beginning at 100Hz this adjustment can be feathered long before the main speakers low frequency begins to drop off. After some listening to music there will be some tweaking to taste. Opposed to a sub simply coming on at a set frequency this result is like braiding the subwoofers to the mains and literally defines seamless integration.
   After this adjustment I have four memory presets with slightly different gain settings. Take the time and get comfortable with the SMS-1 the rewards are worth the effort.

   When I purchased the DD Plus' in 2012 I still had the DD-18 and an EarthQuake Nova 15. Before equalization I ran all four subs and experienced the room loading and the lack of any modes. Using one 12 as a master and running Optimization and Parameters the results were greatly improved. I removed the Nova followed by the DD-18 with no sonic degradation but gained a mode deep in one unused corner of the room. 
   In my limited experience proper sub placement and this level of optimization far exceeds the need for four subwoofers in the two rooms I've used this system in. The variability between extra low frequency systems, rooms, and personal taste is so great I'm guessing there are very few absolutes. Good luck with it.           

db, I forgot to mention Velodyne Acoustics may have suspended their audio related production due to successes with Velodyne LiDAR.  

Keep a list of any third party service providers and print any pertinent manual and wiring information from the site I linked in my previous post. Under Service the site lists two service providers. One on the east coast the other in the LA area. They may be the only access to actual product wiring diagrams.   

Good. Being in California they may be the best knowledge source going forward.

I found a contact in Los Banos who seems to discounting their remaining new stock which seems to be located in Fremont CA. 

Don't forget to printout that Outlaw manual from the Velodyne site I posted above. Its a different format with some useful tips which might be of help. 

   I was unable to get two of my subs in the perfect crawl tested locations but they were close. The Auto EQ did take up the slack in my room.
   I ran my Earthquake, the DD-18 and two DD-12 Plus mastered from one of the 12s. I was able to closely match the equalization of the 18 to the others with the calibration mic at the listening position, individually running Auto EQ followed by a few minor Manual EQ adjustments on the Velodyne's. 
   Your listening space may not need all four units. What sounds good to you is key. There are no absolutes. 

Actually, on page 25 of the Velodyne Digital Drive Plus User’s Manual under the heading of Optimize for Subwoofer Placement, "you place in the room according to the information in the general section and optionally the "Crawl Test" section in the Subwoofer Placement Recommendation Guide" (where the "Crawl Test" is explained in more detail).
As a multiple DD Plus user I can confirm that DD Plus Room Optimization can compensate for time and phase issues that may arise from less than optimal positioning. Its ability in dealing with any rooms unique modes and nulls is far less effective, if at all, compared to beginning with the suggested "Crawl Test".
Sadly, the Velodyne facility is now used for Velodyne LiDAR.

   The SVS Ultra appears to be the first affordable subwoofer with an application that offers needed low frequency crossover related adjustments as well as three customizable memory EQ presets within a single frequency band between 20 and 200Hz.
   Velodyne, JL Audio, Vandersteen, Magico, and Von Schweikert, understand the importance of multiple frequency bands of discreet equalization within a higher crossover region without reducing gain and not simply a set point where the subs kick in.
   SVS claims (SB16-Ultra Owners Manual page 15), "in some cases, reversing polarity - can be used to fix bass nulls or overly boomy spots in the listening area". While it may have a minor affect reversing polarity simply restores the polarity of the recording which is most noticeable in the bass drum and can vary as often as from track to track. If the listening position is located within a null or mode the sub will require proper physical relocation. With the sub at the listening position you don't actually need to crawl around to locate a rooms modes.  
   I'd venture subwoofer sales are related to affordability rather than flexible frequency integration. If they've never heard it how would they know what to listen for? After all, almost any sub sounds better than no sub.