Back on topic, if you wanted to run a sub with the Alesis, you could do this:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-633
1) PC headphone out to SUB: 1/8 stereo to (RCA) sub in
2) Sub to Alesis: Sub out (RCA) to (RCA, 1/8 stereo, or 1/4 TS) Alesis in
On the speakers you mentioned, I don’t know much about them and haven’t heard them. I’d take the Alesis over most PC speakers I’ve heard but this is a limited opinion.
While I was looking to see if the Dayton has a crossover, I saw this deal that you might like to check out as well (caveat: I’ve never heard these but thought the price was in your range):
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-694
About those PC speakers, do a search on this forum and see what you come up with. The higher order members here sometimes comment on such things and you might find out some good info.