[manufelices]

Hi! What do you think is the best portable amp+DAC usb for a laptop? Currently I have the Fiio E 10 and I´d like to improve it. My headphones are AKG K500 (soon AKG 702)

 

[windcqy]

nuforce HDP may be your choice.

 

[NewAKGGuy]

The AKG 701/2s are notoriously difficult to drive so you'll want to look at portable amps which are capable of driving more difficult loads, something with the drive capability of an RSA Hornet at the minimum.
 
Some DAC/Amps to consider
RSA Intruder
Pico DAC/Amp
IBasso D6
RSA Predator
 
None of these are particularly cheap, even used, but that's what you've signed up for!
 
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[DrSheep]

The most cost effective one above the FiiO's will be the AudioQuest DragonFly ($250), follow by the Sony PHA-1 ($350), various VentureCraft offerings ($400-800), than the ones mentioned above.