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.
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I would say mostly for home theatre however I have a old Denon ACR-3000 & I do not beleive it has a internal x-over. I do want to get a newer processor or receiver but it looks like that's a while away. The acoustics in my room are kinda crappy which is why I would like to add a sub.

thanks for the link, I will read up on it
Check for the next Outlaw ICBM-1 crossover to come up on Agon (one just sold for $145..a real steal) or Paradigm X-30 ($100 range).
Active is the way to go. We manufacture a crossover, parametric, and digital switching amp for subs. It also has the ability to switch automatically from music to theater mode. It's only downside is expense, but it is very unique.

Rives sub-PARC
Looks like a great product, only 10x's what I wanted to spend :) You wouldn't happen to have a more affordable non-amplified version would you? The subs I plan to use do not need anything close to 1000 watts.....

thanks for all the input....
Admittedly, it's price is the biggest problem. The crossover that works independently really well is the Bryston 10B or the Marchand (though I can't recall the model that is used for subwoofer application like this).

We considered making a non-amplified version, but you wouldn't like the price of that either--neither did we. It would have been somewhere around $4000, and that's a lot without an amp. The amp is the best we've ever heard for this application--and that's saying a lot. In fact it was the amp that held the project up for nearly 2 years because we weren't satisified with the performance.
Pass labs xvr1