Welcome Bobby.

The gain for the mic is on the front panel. Those 'high and low' settings in the menu are the mic EQ settings. I would put them back to flat, or even roll off a little lows if you sing very close to the mic.

There is a switch for phantom power on the back of the BK9. Make sure it's ON (and turn down the master volume before you change it! Turning phantom power on and off can often result in a VERY loud pop!)

Try singing over one of the factory settings (the ROM Performance List). Make sure you haven't got all the Keyboard Parts at full volume. 100 is a good place to start, or even lower if you play hard!

One thing to check is the microphone itself. Condensers are a bit more delicate than dynamic microphones. If it's been dropped a lot, the capsule might be damaged. Have you used it elsewhere with no issues? SM87's are pretty robust though...

Lastly, what are you playing through?

Hope this helps. Primary thing ito check is the Mic Volume knob (top knob of the second column, just above Mic Reverb). And put those EQ settings back to flat!