With shure 520, you have a dirty sound, but if you uses the shure with a guitar tube amp. better. And maybe you have the posibility to find one Astatic 335, on e-bay, really cheap. with this shit you can make a really dirty harp trash sound.
Im using a shure 58 to record harmonica and everything else, and the harmonica came out fine, although my music isnt very heavy on harmonica at all. If it was Id think the money was better spent in the amp than the mic, but whenever ive seen harmonica players who just play that, they tend to have a fancy retro mic of some kind. As for me I just make sure the recording level is good(around -3)and it comes out fine for a folky sound. If you want that gritty distorted sound like old blues you probably want a small tube amp to run it through ideally. Something should be coming out of your bass amp though, try it with a different instrument and cable to check it.