Optical always gonna sound smoother, but Good decrapifier + Great usb cable outperforms optical.
Optical cant take away and make the sound darker from reference point you have to remember that, the sound you get from optical is what it is. USB adds to the sound so you can use many devices to reduce the noise and try to get it as low as possible to try to get as close to opitical, if perfect isolation is acheived with USB it will sound just like the optical, darker and smoother.
If USB outperforms it to you then that is because of your preference for sound not because it actually gets lower noise than optical which inst technically possible.