Try your audioengine amp and see how you like it. You can email audioengine and ask them about what power specs it has. I think I'd do that before anything. I'd be suspicious of a 'no external power headphone amp'. I assume it runs from usb power. It may be ok for light listening but planar magnetic headphones are known to require some power. I'd look at Schiit headphone amps, reasonable prices and good products. Of course, monoprice also sells headphone amps.