I wanted too know about the same question too and i made a thread about it weeks ago and nothing so im not even sure if you could..
I was using 2 floor standing speakers and wanted too use my Logitech 2300 sub and wanted 2.1 setup but i always had too put it on 5.1 too have it work but i think i was missing on some sounds when its in 5.1.
So i pretty much gave up on the idea and just bought a onkyo receiver from newegg , And got 2 huge floor standing lsi speakers with 12in subs from my local goodwill and just put the 2300s aside, Than a week after i took a old sub from one of my stereo which is 30watts Philips powered sub which sounds pretty good and not too over powering and added that too the receiver.
And than i added 2 more speakers which are rca kevlar bookshelf and put them facing the side of me and the sound is awesome and really clear.
And i admit now that this setup sounds much better than the logitech 2300s
But i still think they are ok budget speakers if you get them under 100$ and for people that loves alot of bass.
But ya still would be nice if it was possible too hook a sub up too the sound card in 2.1, Sense i would use the Logitech 2300s sub for games, But without it the bass is powerful enough. But if i wanted too add 2 subs down the road i don't think i can sense the receiver ony have one preout port for a sub.. So if somehow someone knows how too use the sub input on the xfi without using 5.1 that would be great