Last weekend and again today I played around with my sealed UXL-18s. Last weekend, I tried the HS12 tweak outlined in this thread
but it didn't seem to work for me (post 179 on).
So today I re-positioned the seats (moved them up to ~12' from the front wall), re-calibrated, tweaked and (inexplicably?) things are VERY different and SO MUCH better!
I ran Audyssey with the iNuke in default setting (red line, attached pic), left subs at Audyssey cal (-12dB), then added 6dB of boost at 60Hz on the iNuke and I'm flat down to ~15.5Hz (green line, attached pic).
I ran some demo stuff at around -10dB with DEQ on and the subs are kicking ass! Lots of deep bass and good mid-bass, too.
Given the difference between the levels set last weekend and today, I really can't explain the (seemingly) night-and-day difference. I suppose moving up the seats had something to do with it but, still, it's pretty surprising.
Long story short: For the time being at least, I won't be going ahead with the ported cabs. (I'll keep eng-399's info on hand, though, just in case.)
I will, however, be doing the iNuke fan mod next weekend (a pair of Noctua fans is on the way from amazon.ca).
Thanks to everyone - and especially eng-399 - for their input and help.