The tour I did last month went through a bunch of venues that are old theaters. This is one of my favorite types of tours- a lot of these theaters tend to be in city centers, which offers good proximity to restaurants, etc., and they're usually a really nice size, about 2000-5000 capacity.
But, one of the things that's not so fun is that the FOH mix position is usually stuck underneath a balcony. This means that you can't really hear the whole PA (sometimes almost none of it), and the low/low mid frequencies are probably way louder than they are for the rest of the audience.
This picture is from the Fox theater in Oakland. When I ask people what their favorite venue in the US is, this one comes up a lot. It's great for sound engineers because you do actually hear the line arrays really well, and it's not as boomy as some other theaters. Plus, it's just a gorgeous room with an excellent local crew and a great place to do a show!
And this is from the Vic in Chicago. Also a great venue - but as you can see, the balcony is slightly more in the way for me at FOH. It's not ideal, but as long as I know the difference between what I'm hearing and what the audience hears, I'll be able to adjust accordingly. Under the balcony, that usually means all those boomy frequencies are loud!
Hearing extra low end frequencies is pretty common for FOH positions. This is the same in small clubs where the board is in the back of a room, or in arenas where the FOH is placed in the center "power alley" of the floor. But this is one of those things that you get a feel for the more shows you do.
In any case, I always advise people mixing FOH to walk around the room and get a sense of how it sounds for the whole audience. Even though I know the mix won't be perfect for every person at every show, I just try to get the best balance I can for any given situation.