Okay so here goes, and I'm sure someone else will chime in too.
1. If we take a look at the data-bass.com web site and compare the UXL-18 driver and FV15HP max burst numbers we can see that at 20hz a UXL-18 in a sealed enclosure would have the same amount of output as a FV15HP. Above 20hz the UXL-18 has about a 1-2dB advantage until you get to 40hz, above 40hz it really starts to pull away and you'd need at least two FV15HP's to even come close to one UXL-18 driver.
Below 20hz the FV15HP holds about a 2dB advantage over the UXL-18 until you get down to 12.5hz, at which point the FV15HP pretty much signs off and the UXL-18 keeps going.
2. In a ported design I would have to believe the UXL-18 would outperform a FV15HP if built properly and powered with enough amp.
Here's a graph simulation on a design that I'm looking at using for dual UXL-18's. The enclosure will be 22cu.ft and tuned to 15hz and powered by a inuke 6000.
So if you used a single inuke 3000 to power a single UXL-18 and built a 11cu.ft enclosure I would think you could minus -6dB from that graph to give you a good idea of what a single UXL-18 could do.
3. If you were building two a inuke 6000 would easily power them or take a look at a Peavey IPR 7500, either way you'll probably have to do a fan mod. Which is also pretty easy to do too.[/quote]
So, assuming your model is accurate and I subtract 6 db to get an idea what the UXL will look like in a typical Marty (11 cu ft), then the UXL is only about one db better than the SI18 at 20 hz. I know quantity isn't the only important aspect but I was expecting a bit more of a difference between the two. Check out LTD's model of the SI18 in a Marty box.