Is there DC on the output of the power amp when the preamp is connected?
There are a number of Non-Polarized capacitors is the signal path. One is C1 on the power amp board. These usually become leaky after 30 years of intermittent use. They all should be replaced. Most are 2.2uF, some are 10uF.
Check the collector Voltage on Q11 with and without the preamp connected. Q1 and Q11 really could use small heat sinks or chassis mounting, they run pretty warm.