My very quick review. Both headphone amplifiers are relatively new (Woo WA6 has ~100 hrs) so I'm sure there may be subtle differences/changes over time. I'm using a pair of AKG 702's that also have ~100 hrs on them (some consider these still breaking in). I placed both amps side by side and just swapped RCA cables and the power plug. Initial impression is that the Valhalla performance/sound is VERY close to the WA6. I just placed both volume knobs at the 12 o'clock position and proceeded to listen to some very familiar HDCD's. I would have to say the soundstage was very evenly matched. I know the volume will not be exactly the same with both amplifiers at the 12 o'clock position, but the Valhalla was slightly louder so I tried to match them closely to listen for subtle differences. I would have to say initially the Valhalla sounded a little brighter (less than an hour on the unit) but not to a fault. Bass was equally hard to discern the difference between units. Both my amplifiers are also extremely quiet.....zero background noise. It has been hard getting ample time to do some more comparisons (birthdays, football, etc.) but I will try and get some more impressions as the units burn in over time. Both units sound very good and sonically very comparable. Subtle differences may appear as they age, but for now, I'm happy with both and plan to keep them. I will update my impressions as I listen to more tracks over the next few weeks. Thanks.
Rich