Many good suggestions above... The likely noise source is the preamp itself. Tubes are indeed a possible suspect (if it's a tube pre). One thing I'd like to add is possible dirty/degraded switching contacts within the preamp. Suspect possible noisy / dirty contacts of a PC board edge-connector, input selector switch, tape-loop switch, tone-bypass switch, maybe even your interconnects (have you cleaned your connectors & cables with Kontact?). If you have to go inside the unit, first familiarize yourself with proper & safe servicing procedures, or take it to a Qualified service shop (not all shops are at the same competence level).
