From working with lots of mix engineers and listening in to their tips and tricks, here's one that I've noticed that a lot of them are doing to get their bass sounding tighter. They'll put the whole track into mono.

What this is doing is making sure that the frequencies from the bottom to the top are all covered / not conflicting with each other. This way to can get the low end tight and when you pull it back to stereo you're going to end up with more space in your mix and a really nice, tight, mono, low end. The higher frequencies can then be panned or moved around however you like to have them, but as you have each frequency band covered no sounds should be clashing and you end up with a more open/spacious mix.

So everything should be neat and tidy in the low end, by having it as a mono signal.