Best answer: If you don't measure, you'll always just be guessing. REW is free and the Umic-1 is like $75 - best money you will ever spend on home audio, BY FAR.
Having said that, if you have the room I would go with more of what you have. Adding ported with sealed or even ported with different port tunes is a monster PITA (and, without REW, you'll never know it. You'll just have destruction at some frequency, know it doesn't sound right, and not have the tools to even try to address it).
If the issue you're trying to address is delivering more spl to a listening location that already works, then co-locating more subs will help. If the issue is nulls ("dead zones") then no amount of stacking or adding to the exact same locations will help. You can't power through a null. For a null you're going to have to find a different location/locations in the room that deliver to that location.
Also, is the inuke no longer working? There should be no issue running two new marty's off a new NX6000d and the two old marty's off an inuke. Guts should be the same.
SPEAKERS: 3 x DIYSG 1099's LCR; DIYSG HTM 6's and 8's surround; 4 x RSL C34E's atmos.
SUBS: 4 x UM18-22 Mini Marty's (and always building, currently working on Devastators).
SIGNAL CHAIN: Denon X4400; Minidsp 2x4 HD; 2 x Jensen Iso Sub 2RX; 2 x NX6000d.