[gunhound45]

I plan on using this with my computer. Using some HD558's and a Xonar DX soundcard.
 
1. will I notice any difference?
2. can I power it solely by USB?
3. are there better amps for the same price?

 

[DaBomb77766]

I see no reason why you'd want to use the E5 if you already have a decent soundcard, unless it's broken or something.
 
But yes, you can power it solely by USB and I don't think there really are any better amps for the price, unless you want to make your own CMOY.  Not sure if you'd notice any difference or not though.

 

[MalVeauX]

Heya,
 
The E5 will not do anything likely for you. And nothing in the $20 price range will do anything frankly. Does your DX not amplify your HD558's adequately? I find it hard to think you'd want to spend any money on amplification on 558's--I'd suggest you spend the money on a new set of headphones if you want to have an increase of any kind.
 
Very best,