So, you've got this bass going on in the track. When you're presenting a track to a mastering engineer, you want to put loads of low-end on, and you want to try and get it tight and do your thing on it. But if there's a low cut going on, it's best to leave that to a mastering engineer who can hear what's going on in the low-end. You want more bass and fewer tops when you're a mastering engineer because you want to be able to craft and shape it.
Think of it like clay: if you just add to it in places, it never sticks right; it just falls apart. You can't add what isn't there. You can enhance what's there, but you can't start adding loads and loads since it's just going to sound boomy and horrible. You're better off leaving more bass on because the mastering engineer can turn around to you and say, "Can you turn it down?" or "Can you change this?" If you're making a track, make it so there's plenty of bass for the mastering engineer to grab and shape. I always tell everybody to get the track sounding the best they possibly can in their rooms. But if they are worried about the bass, then I suggest they leave it on.
How do you craft that bass if you're mastering it yourself? If you have loads of bass going on, grab hold of that somehow, but don't grab hold of the rest of the track. The way I do that is with a multiband compressor, with which I grab the low end and sort that out. I'm sometimes mono-ing that signal. If it's electronic music, then yes, it can sound tighter when it's in the centre. But if it's guitar music, then it sounds nice when you add a bit to the width because it can sound thin and start taking away. My thing is to grab it and try and shape it a little bit with some EQ.
The key to a monster bass is to have it there in the first place. If it's not there, ask for some more bass if you're the mastering engineer, because then you've got something to play with. If you're the producer or the mix engineer, leave it on there for the mastering engineer to grab hold of it, shape it. They can always ask you for a little less space if it's mentally over-the-top, or boomy. Then they can always ask for a few tweaks.