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As wimpy as the included subwoofer cable may seem, there's a decent chance that Onkyo sized it correctly for the application. You could try any cable with RCA connectors that you have around the house - a video cable would be an excellent one to try out to see if it makes a difference (because video cables are shielded better than a lot of audio cables). That's what I'd do if you have any video cables or other RCA cables around.
If you are looking to buy, you can get a good quality inexpensive cable like this six-foot cable from Monoprice.com for $4 plus shipping. That's just an example, and they have less expensive ones, too. If it were me, I'd head to Radio Shack, buy one of their subwoofer cables (or any good cable with RCA plugs), and try it out to see if there's a difference. I'd do that because I could try it out that same day and because I could take it back if it didn't make a difference.