This is something I've been messing with for a little while now with mixed results. The easiest and potentially more expensive way to get dirty lo-fi vocals is to buy a harmonica microphone like the Shure green bullet or similar and plug that into a guitar amp or PA. I know through watching their live videos that Thee Oh Sees use this method and I believe specifically this mic. The good thing about this method is that the mic's signal is already matched for amp use so you don't have to worry about volume or impedance issues. Another way is to use the effects loop on your mixer and plug in some guitar effects pedals into it. I've also done this without an effects loop by sending the headphone out to pedals and then putting that back into a separate channel but be careful about feedback. The last way I'll mention is to wire the speaker end of an old telephone handset either directly to a 1/4" cable or to a 1/4" jack and then plug that into an amp or PA. I've seen this used to great effect before, the wiring is super simple and usually requires little to no sodder and it's cheap! I just purchased a NOS speaker element from ebay for five bucks shipping included last week. Good luck!