David you can & should always turn off the PA first in order to avoid the pre. etc. introducing damaging transients upon power down. Conversely, the PA should always be the last component that you power up during a turn-on sequence. Last on; first off.
If the pre is tubed then check tubes & swap them around if possible (power MUST be off) but if solid state then it's likely in need of service. You might want to poke around inside looking for loose / dirty connections if you're familiar enough to do so.
If the pre is tubed then check tubes & swap them around if possible (power MUST be off) but if solid state then it's likely in need of service. You might want to poke around inside looking for loose / dirty connections if you're familiar enough to do so.