What sub x-over to get?

I would like to use 2 subs for my home theatre it's just I am not sure where to look for with a x-over. Do any of you go with passive x-overs or is active the way to go? If active what x-overs are good & can be purchased for under $500 or so. I plan on going with 2 image dynamics 15's in a 2.4-2.6 cu ft ported box. I am also not sure what amp to run, possibly use the adcom gfa-5500 I have now which runs my main speakers & get something better/more powerfull for my main speakers.