That sounds like the Jaycar amp - I've got the same specs here:
http://www.subwoofer-builder.com/amps-all.htm
Xmax is the one-way excursion of the driver, and in conjuction with the area of the cone Sd, determines how much air you can move. To get decent SPLs at low frequencies, you need to move heaps of air.
The Americans are blessed with affordable high excursion drivers, but we Aussies are not so fortunate. Jaycar used to stock a 15 incher CS-2345 with a usable Xmax of 12.5mm but the mongrels have discontinued it!
At the moment, there doesn't seem to be an affordable IB driver for us. I had hoped you had stumbled across one, but it appears not.
If you think you're up to building a hi-pass filter, you could go for the ported design on this page
http://www.subwoofer-builder.com/driver-CS-2246.htm
That amp driving a pair of the 12 inch drivers in a 190litre box tuned to 30hz with a 16hz hi-pass gets you around 117dB peak output.