This sure sounds like your receiver has a peculiar failure mode. Did you say that your replacement receiver does the same thing? Mystery.
How about stereo/mono setting on your TV? Have you selected the SAP by accident? No, you said this was happening on several TVs... If your TV is mono, but the receiver output from SA was suddenly out of phase left/right, then this would explain the symptoms.
How is the audio getting to the TV? RF connection? It seems unlikely that the RF modulator would go bad on two receivers, but it's possible. Many of us use the best-possible quality outputs, which is the S-video jack (and RCA audio) on a 301. If possible, use a TV with A/V inputs and see if those are working. There is less circuitry to fail both in the receiver and in the TV using A/V outputs/inputs.