got a sigma dm2 i’ve had for years but needs some work. a guy at a music shop said some of those sigmas could be worth a few hundred bucks. i’d like to have an idea on it before i try to fix it myself.the serial number is 87110807 on the paper tag in soundhole that also says c.f. martin& co. it has the sigma guitars est.1970 logo on the headstock.any info would be appreciated.