Astro-Sonic channel problem

The left channel on my Astro Sonic seems to be out or low. It is the same with all sources, phono and radio. If I turn the volume up all the way, I get a faint signal coming through. I've swapped out different sets of cables with no change. I've reversed the output cables from the amp, but the left channel is still out. However, when I reverse those same cables on the receiever's input, the bad channel switches to the right. Would this indicate the fault is in the receiver? What problems should I look out for?

If trading cables makes the trouble switch to the other channel, the trouble is earlier in the signal path than those cables are.

If trading cables does not make the trouble switch to the other channel, the trouble is later in the signal path than those cables are.

If trading one end of the cables makes the trouble switch, but trading the other ends of the same cables does not make the trouble switch, one of the cables is bad.

Most likely culprits for no sound on one side:

- bad cable
- cables plugged in wrong
- loose connection
- volume control
- selector switch
- debris fallen into unit
- blown tube or transistor
- blown speaker
- bad pickup (phono only)
- muting switch