Well mainly because I'm trying to limit the box size to around 3 cubic feet and I think the HST-18 wants more like 4.ahh, if you can get it in a trade. why NOT... one of those bridged for an HST 15 would be siiiiiiiiiick (although, if you can get an XLS-2500 why not get the HST 18 ??? not much more and give you a good chunk more output,)
from experience, Audessey does an OOOOOOOK job of setting eq on a sub, but NOWHERE near what REW and a minidsp can do. especially when boosting the low end (which a sealed sub will most likely need)... it's not a big deal, you can pick one up down the road if you're wanting too, but a minidsp or a the dsp in an inuke will trash X32 in terms of it's eq ability for subs.. X32 really shines on speakers, but with subs it's kinda "meh"
Reason being that when I build the second sub as an endtable with the subdownfiring, I dont want it to have to be super tall or really wide and fat where it looks like its something AS an endtable, rather than just an endtable that has a cabinet (going to likely add false doors to it)
Plus the price difference is just something I dont know if I can do (I'm beyond stretching my budget as it stands)... but I will do some modeling just to see...
And the other thing is I had some really nice rings made for the inside of the box that will fit a 15" driver. Its basically a metal ring with threaded inserts attached so it ensures a really tight seal.... So those would be nice to use on the 15. I dont know if I could get a 18" fabbed.