So the noise ONLY accompanies a string pluck? It does NOT make noises when just sitting there on?
Plug a cord into the efects send and connect that output to the input of some other amp for a listen. is the noise on that signal?
Explore the power suplies. The preamp runs on +/-15VDC. Are both present at the ICs and clean? The power amp runs on I don't remember what, but it will be two supplies of 40-50-60-70 volts. Look on the connector to the power amp module - measure there. Are both present, clean, and up to about the same voltage, ignoring polarity?