Summary: Testing setup:
Nuforce uDac > EF-5 > HE-5
uDac > Sennheiser HD 555 (foam modded, with thousands of hours of burn in)
Out of the box, HE-5s sound abysmal compared to my HD555s through uDac. That came as a bit of a surprise to me, as I expected them to sound better even without burn in. I appreciated a drastic improvement after a few hundred hours of white noise, pink noise, and sine wave sweeps.
Pros: Incredibly natural presentation, amazing phones with proper synergy (component matching), easy to replace cables
Cons: Abysmal build quality (wood cracking), uncomfortable design, relatively heavy, barebone packaging, no manual, harsh treble (read more), hard to amp