Waaaaayyyy too easy, but you are correct to some extent. Without plugs or IEM, we're at the mercy of our drummer, who is a pretty aggressive hitter, and we play a lot of hard, heavy music. Ironically, our drummer is the only one who uses IEM, and he loves it. LOL. And he doesn't sing.
So, the rest of us (me on bass + 2 guitarists, all of us are DI using modelers) have to keep up with the drum level, and the FOH PA system is sizeable with pumping subwoofers, playing mostly in rooms of about 300 capacity, and our audience doesn't really expect a polite, well controlled volume. They show up to blow off steam. True warriors, they are.
All this pretty clearly indicates that my best bet is going to be IEM. Old dog, new tricks. Part of the joy of gigging has always been the energy of the show, and the room/PA volume has a lot to do with it. But after 40+ years of gigging this way, it's finally caught up to me and seriously impacting my (solo) vocal performances.
***Mainly, though, I was just wondering if anyone else out there has experienced the same as I have WRT getting pitch-confused due to dominant loud bass frequencies. If I want to continue singing lead vox, looks like I'll have to make the sacrifice and try IEM...of give up on singing lead (which would be fine with me).