Room treatments are definitely a good idea, but I have to believe with the articulation your hybrids offer, the amount of low-end slop in most subs is all the more apparent.
Since you will have both the ML and the SVS in house, you should run a comparison first and if you determine one is sloppier than the other, return it and get a second of the better.
In my experience the correlation between these graphs and a clean sub is very good. If you look at the ML, you will see the sound pressure level is still at 80db beyond 50ms. Frankly this is poor performance for a sealed sub, especially at that price. In comparison, the SVS is below 80dB within 20ms (not the best, but competent, and much better than the ML).
Impulse response/decay measurements by Jim Wilson:
Martin Logan Dynamo 1500X
SVS SB13 Ultra
Had you asked for suggestions of a musical sub to complement your electrostatic speakers, I would suggest the Rythmik E15Hp (or F15HP):
or if your budget is tight, the PSA XS15se:
So, I am betting that you will find the SVS a keeper and return the ML (hope you can).