I just read through the entire thread.
I didn't catch the room dimensions. Is the room sealed or open to other rooms? Any acoustic treatments? Why did you want to add to your existing ML subs? Why add SVS and not more MLs?
The SVS NSD are budget subs. Nothing I would recommend for critical music listening. I'm not a big fan of the ML subs either (way overpriced IMO). I'm surprised by your comments about Seaton and Funk Audio subs (I own both). In fact I helped a local SVS member replace a Dynamo 1500 (which I found surprisingly good) with a DIY UM18-22 w/Seaton amp and there is no comparison.
For your situation I would return the SVS subs, sell the ML subs, and go with a pair of Rythmik Audio subs if you are after musical accuracy. They build the best inexpensive subs for music. I'd like to know more info about your room and your budget.
I suspect you may like the w700s because of higher harmonic distortion (not better harmonic reproduction). The UMIK-1 and REW will allow you to measure this. Moving to something like Rythmik, Seaton, Funk, or a higher end SVS will be an initial adjustment for most listeners because of the lower distortion. You should adjust and begin to appreciate the accuracy and detail provided by higher-end subs.
IME and IMHO, the best bass for music is produced by Transmission-Line (PMC) and horn-loaded subs, the Funk 15.Hs are likely the best horns one can buy (or even build). PMC ATL subs are not cheap and neither are the Funks.
Last, many don't understand just how large are role the room plays in regards to your audio. Take a look at the Lyngdorf TDAI-2170. I believe the TDAI w/RoomPerfect may outperform a 2ch integrated without room correction & bass management. A dealer may be able to offer you an in-home trial.