Are those subs going to be ran at 4 ohms? Are the duel voice coil? If you plan to run them at 4 ohms then I would definitely suggest the iNuke6000dsp. That would be a future proof amp and with the built in DSP you can boost the low end and EQ to suit your taste! Of course, yes, the iNuke3000dsp would also work perfectly fine, and although I am not familiar with the power handling of the IXL-18 mk2, so how much wattage it would take to push it to Xmax, I have no idea. Have you done any models in winISD?
I might also suggest, just for the fun of it, that you go with two ported enclosures over two sealed enclosures! In a ported box, you are going to get a good bit more output around the tuning frequency. That, plus room gain will likely give you just as much extension as the sealed sub, (assuming your room isn't 10,000cuft, but, if it was, then sealed would be a poor choice anyway), and a crap load more at the tuning frequency. Speaking of which, I would do some modeling in something like an 8cuft to 12cuft enclosure tuned to 17hz-18hz or possibly even 16hz. If you went with a larger ported box tuned to around 16hz to 18hz, that would more than likely give you 5db to 10db more output at that frequency, and then you would be getting room gain below that which should be pretty close to the extension of the sealed boxes, but will have much more impact!
All of the above is just theory at the moment as I have not personally modeled the IXL-18 mk2. I am also unaware of the size of your room, as that can make differences as well. What ever you end up going with should be nice as those are great drivers.