6x9 can have some bass. At first you will be happy with it produces as it would be better than ur stock speakers. But later, you will 'tak puas' as the bass is not 'strong' enough.
I started with 2 Alpines 6x9s at the rear, with Soundstream components running at the front doors/dashboard. All powered by my Alpine head unit (forgot what model ##, just a standard CD player with AUX-IN cable for my MP3 player). Didn't focus on a subwoofer coz I felt that the 6x9's produced enough bass for my listening style. Total cost = RM1,200.
1 year later, I felt that the bass output wasn't shiok enough, so I went to Jalan Pasar and bought an 2-channel amp + 10" woofer + box + cables for RM350 total and DIY at home. Not a total bass lover, so just set the bass until I can feel the oomph, but not until it inteferes with my heartbeat lah.
Not bad, not bad. Just that now the subwoofer fills up 2/5 of my boot space.
So now, total cost = RM1,550.
Oh, and RM80 for the AUX cable.
Maybe you can skip the rear 6x9's and go for the sub instead. But then you might feel 'empty' behind your head. I suggest to go for front components if you are deciding between rear 6x9 or front components. More 'feel' in front.