Welcome to the forums!
TBH, the Riviera is a very
old and very cheap sound card.
I doubt any software will be able to save you from cheap components.
If your headphones sound good everywhere else, a new sound card will probably help a lot.
Do your headphones need an amp?
If not, the ASUS Xonar D1
is an excelent option.
It will give you top end sound for a reasonable price.
If you really need to save some cash, the Xonar Ds
will also be a great improvement.
The best option for high end headphones would be one of the ASUS Xonar Essence
series cards but they are not very cheap...