From one response on your link it seems that I was wrong about the power plug. One poster claimed it was the answer to his problem which appears to be the same as yours. Try the plug, if it doesn’t solve your problem you can always return it.
The D power plug is the wrong tool for the job. It’s designed to drive long lines from a dynamic mic. Try your mic w/o the plug and see if there’s a level difference. Is 48v phantom required for your mic or is it internally battery powered?
Your approach of setting the board for meaningful levels and then raising the amp level for room volume is how I have done it. Same as setting up your mixer and adjusting the headphone level for your comfort.