The E7/E9 combo is nice because you can really use any headphones with them. I say this because in my experience with this amp and low impedence AT headphones these amps simply magnify and charge the sound that the headphone would normally be putting out. If you have a heavy bass headphone, well that's going to be magnified. If it's muddy somewhere, it's going to glare. This amp doesn't necessarily make it any warmer or cooler.
For example, before I modded my W5000's the mids were very aggressive and fatiguing. This is because they are naturally this way with this headphone and the e9 made it even more aggressive.
And under 500 dollars for a budget? You can some of the best cans for that much. If you like open full size cans, check out the AD1000's. Plus they'll match the amp.