How to integrate a sub with 2-channel mains?

I've done a ton of searching and haven't been able to come up with an answer to this... Hopefully some of you can help!

I currently am running a DIY BP1503 for my home theater/television, and it is everything I want for that. Howerver, I want to get a second sub for 2-channel music; probably a UFW-10, something much "faster".

What would be the best way to integrate this with my mains? I've seen many people post that they did this, but as near as I can tell, they are not using their processor to xover the mains and sub... How do they hook these up and cross them over?

Thanks in advance!