I'm thinking of connecting Empeg to my Sony C8900 throu an Sony XA-300 or to get a "aux-switcher" that let me connect both the sony and the empeg directly to my amplifier. What should you do?

Well, is you are satisfied with your Sony HU sound quality then connecting empeg to its aux input is something to be considered, mostly due to simplicity. Of course, the other solution gives you more freedom and flexibility in designing the rest of the system.

BTW, contrary to many old hands here, I think that having another HU configured to be able to work without the empeg is a good idea (for short drives to the supermarket when one doesn't bother to bring empeg, for drives to destinations where one doesn't want to bring empeg, when empeg is digesting a long sync, to hear a new CD on the way home...)