You can - just buy two speakers at a time
In three months you can have a 6 speaker surround sound system...
Orb Mod1 Quick Pack - $239
It'll take you two months to get the 8" Sub - but at $400 for a sub I might be tempted to also look over here: Open Box AV123 MFW-15
Edit: or maybe not the MFW-15 since apparently they catch on fire or some such..... But there are certainly other subs that could be nicely matched with Orb speakers in the $400 price range.