This is definitely not my experience with these mics. I wonder if your SM7B is broken or has its phase reversed.
I did a shootout (not on Youtube...) in my homestudio with the following mics for guitar:
-SM57
-MD421
-RE-20
-M88
-M201
-SM7B
It made me sell the EV RE-20 since it was the most polite and boring one (also since I have a few lovely kickdrum mics... vintage AKG D12, Audio Technica ATM25, no harm done there) and while the Beyer M88 was the clear winner for me in that shootout, the SM7B was warm, thick and smooth sounding. Totally unlike your results.
I think I used either a Warm Audio WA12 or a GAP PRE-73 as a preamp, FWIW.