If there is not a sub recommended by Fostex for the NX-6a's then you are just going to have to go with something that works. I use a 10" sub that doesn't match any of my monitors. Really, all you are doing is making sure you are covering all of the frequency spectrum. I'm not sure I would recommend a sub in an untreated room tho. It can really cause more problems if the room is already lying to you.