Preamp, Sub, and Magnepan connect


I have a pair of Magnepan speakers and I'am considering connecting a JL audio sub with it's own power supply. What is the best way to make the connect if my tube peamp that only has 2 channel output? Do I modify the output from Preamp?
salc
thank you for your immediate response, This chat forum is great. One followup queston to Macdadtexas, Are you suggesting REL instead of JL audio?
I think you will find the Rel subs to be faster and a better match for your Maggies. JL's are pretty fast and hit hard but Rel is known to be a good match for Magnepan speakers.
I don't own a Rel, but I think they are the best all around match with Maggies that I have heard. That is not a knock to the excellent JL Audio subs, which I have had in my system, I just think for what you are looking at doing the Rel is a better match.