What are you plugging the amplifier/HD 800 into? (What plays your music?).
Do you subscribe to the school of thought that all gear is explicitly different sounding and that price correlates to sound quality, or do you subscribe to the school of thought that there isn't really much of a difference after you surpass some quantifiable boundary?
No matter what you say to that question, the Woo amps will very likely be inherently less linear than the Asus (on a scope), so they should produce some audible differences. The WA2 is generally reviewed as better, but "how much better" is probably open for debate.
Within your price range, I would probably 86 the Asus simply because it's a $600 TPA6120 amplifier (at least TMK) - sure, it has more features than some *other* $600 TPA6120 amplifiers that I can name, but you can get a TPA6120 amplifier for around $100 on Amazon (and it's called the Fiio E9, it works great). There's more exotic amplifiers available on the solid-state side within that price range, or within the price range of the WA2 (which is the most expensive of the three, by far), if you subscribe to the first school of thought. Otherwise, I would suggest finding something with good channel tracking and sufficient output power to handle the HD 800 (they're not the most demanding thing in the history of time, but they're not DT48A.00s or Beats Pros either). If you want a 6120, Music Hall has a nice model with two outputs, there's also the CIA (but that gets into the price range of the Asus...and lacks a DAC). There's also some other absurdly over-powered SS amps out there, like the Musical Fidelity M1.
Oh yeah, and Sennheiser is releasing an amplifier to match up with the HD800, it's supposed to cost around $2000 (and honestly when it makes the m903 look like a value alternative, I'm very skeptical).
