I tried a "Woodpicker" on my guitar (D-28) and it sounded pretty good. I have a stick-on pickup on one mandolin, bot a Woodpicker, and it's not bad, but only so-so (another "A model," but by Harmony). I may try the woodpicker on the mandolin soon, since I've taken it off the Martin guitar. The woodpicker has a block of dense wood on top of the pickup, and it doesn't seem to feed back as much as the other one I have. I'd seriously give one of them a trial, since they ain't 'spensive (I got mine on Ebay for $18, but I think they're normally around $40 or so.

Bob Clayton