Agree with your diagnostic: your customer is crazy
Only possible with a 2 mile long microphone cable.run harp through the dirty channel with the gain cranked and no feedback.
Truth is *all* killer harp amps are real old Tweed era amps, or same vintage PA amps, with basically flat frequency response (very little eq if any) , gain enough to barely reach clipping but no more, and bass and treble limited by their cheap construction.
The end result was good ... or we are used to it because that's all there was available in the "golden era".
Frequency limiting and shaping came also from the very poor dispatcher type microphones used, plus using them in the terrible acoustic conditions between cupped hands and the like.
With due respect, changing any ingredient in that classic recipe will mean failure of the dish.