The Manual for the SM48 shows it with a 3 pin XLR connecter. I think one of the problems you are having is actually impedance matching. Either buy or borrow an XLR male to XLR female cable and try plugging the mic into the 3 pin input. It will probably make a huge difference.

Another thing to try is running and electric guitar with a distortion pedal stomp box into one of the inputs and seeing how that compares volume wise with your 12 w practice amp. At worst it should be easily loud and at best it will be much louder. If it is not the amp is probably in need of a service.