Why would you pay for cables with banana plugs attachedd only to cut thenm off?
Besides, as Nick said, all a sub needs with most HT receivers is a single shielded interconnect with RCA plugs on the ends. (and yes, you DO need those RCA plugs)
I hope you didn't cut the ends off, have all packing materials andthe receipt. Get yer money back and buty an interconnect ...from a different store.
They either don't know what they are talking about or they are lying through their teeth in order to suck your money outta yer pockets.