Maybe you just hate mp3s and truncating to 16 bit? Plenty of us go out on a limb and say we're bugged by truncating to 24 bit. Truncating to 16 bit ought to be enough damage that it could easily account for this, IF you just happen to hate what it does. Some are more sensitive than others, and your talk about what you like with the in-DAW sound (going to 24 bit converters, I assume?) suggests maybe the stuff you want out of audio is stuff that dies from truncation.
When you're talking about texture, truncating to 16 bit is NOT subtle. It's very much texture where you hear issues with truncation to 24 bit (and at that, not on every listen). If you're making 16 bit truncated files there isn't that much more to lose to get to 320K mp3, plus it will throw away some of your truncation artifacts for you (it'll go from crunchy, to mushy).
This is free, ought to work in Cubase whether it's PC or Mac, and is designed for the purpose you're doing. Use the 'CD' version for going to 16 bit. Might help. (I also have TPDF, but it defaults to 24 bit because I'd rather use the fancier one for 16 bit stuff)