Hmmm, I get different results in winISD. My numbers show the IXL in a 4 ft^3 enclosure (Qtc 0.796) is down by 4 dB as compared to the UXL in the same enclosure (Qtc 0.582).
I calculated at Xmax for each driver, with LT (f0=35.71; Q0=0.58; fp=20.00; Qp=0.71).
If one looks at the maximum SPL tab which does not factor in EQ, the IXL isn't far behind the UXL at 20 Hz (-2 dB) and exceeds the UXL from 35 Hz and above (by less than 1 dB). Depending on amount of EQ I can see where the are pretty much the same. With a different enclosure size this difference may be even less. $400 is a lot of cash, 3 for less than price of 2. 4 of the IXL's would be pretty cool.
Current Gear: PJ – BenQ PE7700; Receiver - Onkyo TX-SR805; Blu Ray - OppoBDP 103; Turntable - Dual CS-515 w/ Ortofon Super OM10; Amplifiers - OdysseyKhartago, Adcom GFA555, QSC RMX 1850HD; PEQ - Behringer FBD2496 [Near FieldSub], miniDSP 2×4 [Flanking Subs], REW; Speakers - 4 Pi (w/B&C DE250/JBL2226H) × 3 [LCR], Yamaha Crap × 2 [surrounds], Exodus Audio Maelstrom-X18" sealed sub [near field sub], LAB-12 based ported sub tuned to 22 Hz x2 [flanking subs, XO set at 40 Hz]