Sigh.... RF modulated video is about as piss-poor as video quality can possibly be. DVD and digital cable will be severely affected for quality by running the video modulated into the TV.
If you don't have a DVD player, but just have a VCR/Cable, then hook the VCR and cable directly up to the TV via the coax. Then take the audio out of the VCR & cable box and run it into the receiver.
If you have a DVD player... well... Really, REALLY, time for a new tv that has at least the yellow RCA jack in the back.