I have C15 as my preamp, for the power amps I use two MC2205’s driving a big JBL Studio monitor bi-amplified. After an extended non use period I fired up the system, the high end was working fine but the low end was dead, the solution was simple, I started turning various knobs on the dead power amp back and forth and boom it started working just fine, try this before you consider more drastic measures, who knows it might just work.
My guess is that build up of corrosion in various moving parts can cause this.