I am going to try a shot in the dark here, but I would wager to say that a cable signal is a cable signal, unless it's HD cable.
I would also venture to say that you should watch TV through the receiver in 'Stereo' mode, and not surround mode (unless the station sends a signal for that sort of viewing -- such as HD cable stations).
I generally watch TV through 'stereo' mode, and most channels are exactly the same. Maybe you could get a cable 'amplifier' which would boost the cable signal a little, thus, increasing the sound...
Again, that's all speculation...I am no cable TV guy...but routing the cable through the receiver works for me.
Good luck...hope that helps.