Sir
Do you have the HX6946 or V6946 ?
For info purposes only :
For producing bass, SUB is used, even a 8" SUB will make more bass then 2 pairs of 6 x 9 speakers.
If you dont want to install a SUB for any reasons, there are other speakers which can increase the bass and output significantly, example.
Alpine Type R
Alpine Electronics of Asia Pacific Co., Ltd.
JBL GTO 928
JBL
AMP is not for SUB only, AMP will give you power and magnify the sound signal.
Currently your Head Unit/ Cd Player is only giving you 10W RMS of power to all 4 speakers at full volume.
Means, if you want to drive your speakers at 60-80W RMS or 400-480W PMPO, you definitely need an AMP.
Example
Kenwood M845, which will drive your 4 speakers with 60W RMS or
Kenwood - KAC-M845
Secondly, if you want some decent sound at front, 4 inch size is too small, you have to remove the stock speakers and and install 6.5 inch round speakers at door panels or kick panels.
Example
Pioneer 1683S
http://www.pioneer.com.sg/storefront2/aspFn/Product.asp?ProductId=701&CatId=2,27,39
Alpine SPS 600
Alpine Electronics of Asia Pacific Co., Ltd.
Sony GTX 1640
XS-GTX1640 : Speakers : Xplod Speakers / Subwoofer : Sony Middle East & Africa
Remember,these speakers will add bass but only comparatively and they would sound a lot better with AMP, regardless of low volume or high.
Even your current speakers are only operating at 15% of their potential or roughly around 10W.
Regards