I'm excited about the hugo but let's not get carried away. The amp section is just adequate compared to desktop amps. I actually liked my peachtree nova headphone amp better than Hugo's. Seems on the high end for mobile amps, but I haven't really had any experience with other portable players/amps aside from idevice which I always thought was crap.
Some headphones are easier to amp than others and synergy also plays an important role. Just because the amp is only adequate for a Sennheiser HD-800 or a demanding orthodynamic, doesn't mean that it won't play well with other headphones which aren't so picky. Of course a portable amp is going to be restricted in terms of current output as the power supply in a lot of full sized amplifiers is bigger than the Hugo itself. I wouldn't buy a Hugo if my sole intention was to drive a planar headphone to its full potential (even though it is quite powerful for a portable). My experience with Audezes and Hifiman headphones has taught me that they need gobs of power to sound their best - even the Audeze X series which are touted for their so called efficiency.
For Grados, Ultrasones, Fostex etc, I'm sure the Hugo's amplifier will be more than sufficient. May I ask which headphones you've been using with your Hugo?