Not all Receivers are equal.
I had tested a old crappy sony 5.1 receiver once before, and it was horrid compared to the yamaha I had. Lacked in the lower end, almost sounded like a tin can.
Onkyo Receiver have been known from time to time to sound more hollow, especially the lower end models.
Really not much you can do. It is a fault with the Receiver's weak AMPs and undersized power supply. Most likely it is limited by how much power can be feed into each amp.
If you can, send it back and pickup a Yamaha. Yamaha doesn't mess around with crappy sounding receivers. Denon make some good receivers as well.
Also a Powered Sub may help, but wont fix the real problem.
Also ^ Yes make sure you don't have any active crossover active on your speakers.