Definitive Technology BP2000 Woofer replacement

I would like to improve the low frequency performance of my BP2000's by installing better 15" (sub)woofers. Problem is, I don't really understand what I need to look for in a replacement woofer. I have the Thiele/Small parameters for the original woofer from Definitive, but I don't know how to apply them. If anyone has done this with good results, I would like to know what woofer you used. Or if someone could point me to some good references on speaker (or subwoofer) building, or explain to me how to apply the parameters. And, which parameters will need to remain constant, such as power handling and resistance (ohms). Any help here is greatly appreciated. Thanks.