Hi there. Bought a second hand bongo 4 Hp 2 days ago. tested it and everything was fine. At home swapped strings with fresh ones and suddenly a superhuge different volume issue only with the piezo pickup appears. Low E string has huge volume. Other strings seems almost dead and heavily 'compressed'. Fresh batteries didn't help. Lowering piezo trimmers neither. Sort of clueless and uncomfortable feeling now. Any idea appreciated. Regards.