With the Heron volume level set to 60 and the M22 gain set to Low, the sound was hollow. With the Heron volume level set to 60 and the M22 gain set to Mid, everything opened up. The detail is there, and the sound is so much more musical with a wider soundstage than the M12. The M12 is a good piece, but as my system has evolved, the Heron fits soooo much better.
While the hiss is still slightly there, it is less with the M22 gain set to Mid. I’m satisfied (at least for now).