Depending on the output impedance, it might be a good idea to toss both your amp and DAC out and replace them both with the soundcard. The Woo has low THD as far as tube amps go, but it's still orders of magnitude higher than the Essence STX amp's rated amount. SNR is considerably lower on the tube amp. If the output impedance is higher than about 11 ohms, it'll be coloring the frequency response of your headphones more than the soundcard will, and that's assuming the Woo has a flat frequency response (which it might not).
The Woo 6 puts out 580mW into 32 ohms (your Grados). That's more than overkill, a completely unnecessary amount of power. It puts 550mW into 60 ohms (about what your Sennheisers and Superlux are). That's just as much overkill. That's enough to drive a Hifiman HE6, the hardest "normal" headphone to drive, to over 110dB, enough to damage your hearing in under two minutes per 24 hours. It can drive your Grados to about 125dB or so, enough to cause almost instant hearing damage (and probably enough to blow the drivers, too). You don't need that much power, so you're safe to "downgrade" to the soundcard for better measurements. That's provided the Woo 6 has an output impedance of 11 ohms or more, about what the soundcard has. Another amp entirely would be a nice idea, too.
As far as upgrading simply for the DAC, I wouldn't bother. The Musiland 02 looks good from its published specs. Not as good as the Essence STX's DAC, but the difference will be inaudible if the other unpublished specs are also up to the task.
If anyone knows the output impedance of the Woo 6, please tell.
Edited by Head Injury - 9/9/11 at 2:53pm
