Suggestions for 3.1 bookshelf speakers under $300

You really need at least $300 for the subwoofer alone. If it were me I would spend the $300 on the front L/R if possible.

Option 1 ($300 on just front L/R)

Ascend Acoustics CBM-170SE ($298 during sale)
Cambridge S30 ($219)

Option 2 (3.0, 3.1, or 2.1)

Front L/R: Pioneer BS22 ($89 on sale at Best Buy)
Center: Pioneer C22 ($99)
Subwoofer: BIC F12 (would put you over budget), Pioneer SW-MK2, or Martin Logan Dynamo 300 or skip completely and save for a better subwoofer from HSU Research, SVS, Outlaw or Rhythmik. The Pioneer and Martin Logan options are really stopgap. Neither extends very low but would do better than your bookshelf speakers for bass.

Polk CSR $50 x3 for your LCR

http://www.newegg.com/Product/Product.aspx?Item=N82E16882290041

Dayton Sub1000 $120

http://www.parts-express.com/pe/showdetl.cfm?partnumber=300-628

I would look at the Mordaunt Short 10" subwoofer
http://www.accessories4less.com/make-a-store/item/MORDCARN9CAL/MORDAUNT-SHORT-Carnival-9-10-100-Watt-Powered-Subwoofer-Calvados/1.html

And 3 each NXG Pro 4.1 speakers
http://www.amazon.com/Nxg-Pro-4-1-Satellite-Speaker/dp/B0044UHVOK/ref=sr_1_1?ie=UTF8&qid=1357615683&sr=8-1&keywords=nxg+pro+4.1