Maybe tilt the amp up so you're hearing the speaker on axis. The highs drop off rapidly as the angle increases. What the mic hears on axis might be quite different from what you're hearing at 45⁰.
I've had an SM57 on the grill cloth of an amp that was painfully loud from 20' out. I think the mic will be fine.*
If this has already been said, it bears repeating. The amount of distortion that sounds right in the room can be too much in the recording.
My default mic position is about a...
That's not what I'm saying. I'm suggesting that you see how it measures and sounds at different positions in order to narrow down likely causes. If it sounds and measures different in different places, it's most likely a room issue. Eq isn't a great solution to that problem.
The first thing to do is determine whether it's a room issue or a speaker issue. A large peak at 141 sure sounds like a room resonance or standing wave to me, and the best way to deal with that is acoustic treatment.
Some DAWs (ProTools, Reaper, Vegas Pro) can play video and record new audio tracks alongside it. That would solve the real-time sheet music issue. So you would download the video, load it into the DAW, record your practice performance as a new track while listening to and watching the video...
The size of the waveform doesn't matter, but one way to do that is have some percussive sound with high peaks but a low average level. The peaks will visually merge and make the level appear high when you zoom out.
If you're using the mic that's provided, the gain is probably already set close enough to what you need. So that's mostly just a matter of bringing it up in your ears. Start your receiver turned down a bit from maximum then have him raise the mic level while you sing until it's about right. Then...
Only the left inputs of the stereo channels are mono. They're connected internally to the right channel inputs by default, but that connection is defeated if you plug something into the right input, making them stereo. You could use either left or right output.
An aux send is typically mono...
Bizarrely, the text of the manual doesn't explain much about the stereo line inputs. All the other connectors and controls seem to be covered, but all they say about the stereo inputs is that there's a +4/-10 operating level switch. I had to go to the schematic diagram to find out that they...
It really depends on what you're doing with the mixer. If it's for live use, a pre-fader aux might be preferable. If it's just for home rehearsal, you only need one mix (main out).