Mixing in sub with Swan M200's?

The M200's only have one composite input (no speaker-level) and my main question is what would be the best way to mix (connect) a sub in with them?

http://tinyurl.com/a4tho

The speaker posts pictured feed the other speaker. You would need to input into a sub from the PC (Mac in my case) then into the M200's. This would help to integrate a crossover point as well. I don't think a speaker-level connection would work unless you use a separate amp/preamp or integrated setup. That's what I'm trying to avoid since I'm only working with one source (my Mac). I want to use the included amp in the M200's and sub if possible. What's a good (under $250 new/used) sub with preferably a composite input and output that would crossover as well?

Scott

I am also interested to know as I intend to purchase the Swans for my bedroom computer. I own a REL and will try the speaker-level connection; a regular line-level connection sub should work equally well.