Dude - your preamp outs are line level RCA connections - you can't run speaker cable from them. You run interconnect cables with RCA connectors from the preamp to the sub - one right, one left.
Alternatively, if your sub has speaker connections (aka binding posts) as well, you could run a second set of speaker cables from your amp's binding posts to those on the sub.
Don't use both methods at the same time by the way.
If your preamp did not have the extra set of outputs you could use the speaker cable connections outlined above (assuming your sub has spaeker connectors or create a virtual second set of preamp outs via the Y-cables that Rlwainwright mentioned.
The jury is still out on what connection method is best.
