audio video receivers are, ultimately, going to yield you limited power distribution, as compared to better separates based systems, everything considered. Not knowing your speaker models - unless your main speakers offer at least powered woofers - my first thoughts are you will get advantage in running whatever speakers you have at 80hz on your receiver (even if they're full range), and letting a powered sub do the bass. Dynamics will most always improve greatly, and the system will be much more efficient, less taxing on the receiver to be certain. Yes, you must ALWAYS anyway consider making sure ea speaker is placed properly, and the system set up well as a whole.
So, yes, I'd focus on making sure the system is set up to maximize the sound, so do your research here.
Also, adding an outboard amp will greatly help your situation.
BTW, does your receiver have audyssey? If not, the newer will do better for your acoustics in helping smooth things out (remember to not place speakers in acosutic "holes" to begin with, as you can't fix that with an EQ). It's a thought. Staying with things as is, it's settings, speaker/seating placement and acoustics, basically. Adding better amplification yields an easy 100% improvement in soujnd, most always, as well! However, to be true, running your speaker as small does make for a much improved sound on it's own, as opposed to running full range.
What model speakers again???
Another thing is to amek sure you have matching speakers up front, at the very least, and then ideall matching all around if you can.