If the power switch on your Threshold S/300 amplifier is malfunctioning, you have a few options for replacement:
• Contact Threshold Audio directly: They offer service for vintage models, including replacing worn-down items like the power switch. You can reach them at support@thresholdaudio.com.
• Consult Jon Soderberg at Vintage Amp Repair: He is a former Threshold engineer known for repairing and upgrading these amplifiers and may have replacement parts.
I hope this helps - Threshold Audio (manufacturer): They offer full restoration and repairs for vintage models, including replacing worn parts like power switches, capacitors, and RCA jacks. Contact them directly at support@thresholdaudio.com for service and pricing.