If you are thinking of getting a separate amp, I would defiantly recommend changing the card.
Not really for the higher quality sound, more for the onboard amp and the combine price.
If you want to get an amp that can match/beat the Essence's you are looking at an easy $400+....
Even with the amp set to it's medium output setting, my AKG K701's sound amazing and are quite loud at 25% volume.
I just checked
Head-Fi's Headphone Buyer's Guide and I do see the Beyerdynamic dt990 ($200 to 300 range, Open Headphone) as a recommended set of cans.
They defiantly DO need amplification to really shine.
I would be willing to bet that, with proper amplification, your cans really are quite good.
Even if they are making noise now, they really are not making the sound they are intended to with out some real juice!