I am using an SM 58, an SM7b or an Apex 460 thru a BR800 as controller/preamp.

I am assuming the BR800 is a preamp as it has a ton of FX and settings that can boost and change input/output.
1. I need to turn down the backer tracks on Sonar to almost nothing to hear the mic's vocal input in the cans.
2. Tweaking the various pots and fader I can hear the mic start to sound loud enough, but by that time, it's overpeaking the BR and Sonar's meters.

There are 3 volumes that affect the input meters.
1. The mic input sens. (Using #4)
2. the Master Record level
3. the Master (L/R) fader

3. there is a USB setting that seems to help but I'm not sure if I'm using it right?
The list is
Direct Monitoring (On)
Input level (0 dB)
Output level (-12 db) (lowering this seems to help but is it the right thing to do?)
input mix (on)

Without studying the Br800 more than I can during football, I can say that the preamps are probably the same as the roland/Cake gear.  Like many interface preamps, they don't provide a lot of gain in comparison to external ones.  I would think this was the main problem, except you say you have to turn down the backing tracks.  Something is a miss in the gain structure coming out, not in.  Output level should be fine - it is what is going to your speakers.  But you shouldn't have to.

Your best bet if you can't make head or tail of the manual is to set everything up and just roll though the settings.  Sorry, not much help.

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