Your Phase Linear P3800 was introduced in 1983 ($650) and is therefore 30-37 years old. During this time, potentiometers become dry and scratchy and with continued use become non-serviceable and must be replaced. Additionally, the electrolytic capacitors have reached their end of life (they have a nominal life of 15-20 years). The electrolytic capacitors must all be replaced, if they are not leaky or dried out yet, they soon will be.
Your preamplifier was the top of the line in the early 1980's and had a superior phono section compared to the less expensive P3600 ($450). The P3800 also included an input for CD devices which had been introduced to the USA the year before in 1982. It is worthy of a complete servicing and there are many folks here on the Forum that can do this for you. If you are adventurous, you can attempt the work yourself. Just be careful because installing one electrolytic capacitor backwards can wipe out an entire section of discrete components.
Good luck with your P3800.