to get the best bass out of smaller speakers your best bet would to go with a set of amplified components.
some will tell you that 6x9's dont sound as good as 6.5" because the irregular shape causes distortion. although this may be true, if all you want is more bass, the 6x9's obviously have more surface area and have ability to sound louder than a round 6.5"
imho i would tell you to get an 8" sub and amplify it, but since you clearly state you dont want one the alternative is components, but they will still need to be amplified.
basically if all you do is replace the speakers without amplifying them, it will not be
any louder. and if you dont want a sub, because you want an easy install, then the components are not the answer.
an 8" sub is super easy to install, cheap, and will sound great. rear 6x9 components are somewhat pricey and IMO are not worth it.
but hey lemme see what i can find you
EDIT:
6x9 components are expensive and not that great, but here you go:
http://www.crutchfield.com/S-WeNZocm...50&I=049RCE269
2 channel amp for them:
http://cgi.ebay.com/ws/eBayISAPI.dll...756812872&rd=1
or
http://cgi.ebay.com/ws/eBayISAPI.dll...757112467&rd=1