HEADLINE: CHECK YOUR INTERFACE!!!!
I just had this issue, and it was driving me nuts. Like, literally crazy. I'm finishing up a project and have been in the mixdown phase. The only thing more disheartening than your exported mix not sounding right is your 50th exported mix not sounding right after your 50th attempt to fix the issue to no avail. I was on the verge of quitting, ready to just go back to hard disk recording and say screw DAW recording. I had had other substantial issues before, too, so after a certain number of rounds of doing more tinkering with the software than recording, you start to lose it a little.
I'll cut to the chase- it was my interface. I've been using an Audient iD14 interface with Studio One Artist for the last few years. I had had some other issues- crackling, sound cutting out, etc. and kept trying to figure out what it was. Then I tried exporting the mix of this latest project out, and it sounded like the bass was practically gone, and the mids were seriously compromised too. The mix sounded high and tinny- on the same headphones I had just used to mix in Studio One. I tried everything to fix it- I checked my sample and bit rate (setting them to 44.1 kHz and 16 bits), turning the dithering on and off, turning the plugins off and just exporting the raw files, exporting with real-time processing, uninstalling Studio One and reinstalling, uninstalling and reinstalling my interface drivers- I tried basically everything. Nothing worked, and I was hitting the wall after something like 12 hours of banging my face up against it.
I went into Guitar Center and spoke to a very nice associate and fellow engineer Miguel, who told me that my interface and its drivers might be outdated. I bought a Steinberg UR12 from him then and there, took it home, plugged it in, and guys- I am so happy haha. My mix sounds exactly as it should when exported, the same as it does in the DAW.
Don't lose heart! If you can, try replacing your interface, even temporarily with an inexpensive one if need be. If that's not the issue, try some of the other things that I did and that have been mentioned in this thread and see if they work.
Cheers, and the best of recording to you!