You would likely benefit more from a sub.
1. Sound stage is exspanded with a sub.
2. 50-30hz is filled in giving you that all important kick for drums and such.
3. Movies just aren't complete without a sub that can atleast run strong into the mid 20shz.
4. You can find a pair of surrounds on ebay for almost nothing. I use some old blose for surround duty in my bedroom ht.
Wharfedale wh-2 bipole surrounds are cheap and do a tremedous job.
I run my Rocket 550s large with twin xsubs holding down the low stuff. The 550s are pretty strong into the high 30s but the sound is so much more enveloping and enjoyable with the subs.
If you are receiving a sizable refund than go for an Aci force or an Onix ufw12/ufw10 from av123.com (the x-subs rock also and are cheap). If you want more emphisis on movies (sub 20hs) hsu or svs.
Onix ufw12 - 799$ without rdes - 22-23hz -3db
Onix ufw10 - 599$ 25hz -2db
Aci Force - 750$ 20hz -3db
Hope this helps,
JR