You're not missing anything if you don't know what you're missing. Most receivers make better amps than portable amps. As far as if dedicated desk headphone amps are better than receivers, that all depends. It all depends on how you like your sound colored. Receivers are very good as headphone amps. Some dedicated amps will be better than receivers, but some will also be worse. Don't feel the need to upgrade just "because." Only upgrade if your receiver has sonic flaws that you know can be improved by new gear.
