The Eminence Kappalite 3015-LF is a nice light weight bass driver. If you are looking for more chest thumping kick, the Definimax 4015LF seems to provide that. It has a bigger voice coil and I believe a more rigid cone. The Kappalite series are designed primarily so bass players can reduce the weight of their rig in a 2 x 15 configuration. It seems more emphasis was placed on lighter materials (neo magnets, lighter spoked cast frame) than outright performance. The definimax line is heavier, has ceramic magnets and great xmax/xlim.
Here's a really inexpensive cast frame driver I am tempted to try some day. I own the 10" version and its not bad at all. The 15" version has all the making for a decent OB bass driver, provided the T-S parameters are accurate. My driver measured a variance of around 5% from the published specs, not too terrible.
http://www.mcmelectronics.com/product/MCM-AUDIO-SELECT-55-2983-/55-2983With a coupon code you can get these for around $66 each US. By comparison, the Kappalite is around $250 US if memory serves. These MCM drivers have a cast frame, 4" voice coil, 7mm xmax, .46 Qts. and a 99dB efficiency. If the accompanying graph is accurate, they are still producing around 95dB at 35 Hz @ 1w/1m, right up there with the better Eminence drivers. The customer reviews seem pretty favorable as well.