The Pioneer 9500 II I looked up on the web shows it's around a mid '70s origination. If that unit is that old and never serviced, the power supply capacitors may need replacing. If the unit is sounding intermittent, it could be the amplifier power is not enough for the speakers you are using, hampered perhaps by the old power supply capacitors. Or if you're running both sets of speakers at the same time, that may be overloading the amp?

The Theta unit should be pretty good, and is also older. May need some minor fixing too. But if working fine, it's pretty capable for audio for video viewing. The HDMI may be a bit dated, with newer HDMI versions. Get a manual via download and see what this unit does.