M1A, problem with feedback
About one month ago I purchased and installed an M1A pickup in an older, but otherwise quite normal, Takamine acoustic guitar. I am experiencing serious problems with feedback. Two kinds occur. One appears to be a harmonic high. The severity of this type depends on the note plucked. The second is a strong low pitched form that seems to come from the body of the guitar. I can stop it by pressing hard on the guitar body. I have tried several amplifiers and a PA with identical results. I have repositioned the pickup from the fret side to bridge side of the sound hole, and have tried different angles. I messed around with the height of the poles. I have installed several different thicknesses of felt, cork, or paper washers to isolate the pickup from body vibrations. Only very thick (~1/8") washers have any effect, although they do not solve the problem. It returns if the volume is elevated.
At present the pickup is unusable to me, whether that be in a live performance or home rehearsal setting. Is there any reason to believe that there may be a problem with this particular pickup? The reviews of the M1A have been uniformly good, and its feedback resistance is often mentioned.