I have never heard either of these Woo amps, BUT, if you plan to use Grados, I think the WA6SE is the better choice, since it is transformer coupled and as such will do better with the very low impedance Grados. The WA2, while undoubtedly fantastic sounding, is an OTL (output transformerless) amp, and so is better, generally, for high impedance headphones. It may well sound great with the Grados, but it is still asking it to do something it isn;t really good at. At least on paper, the 6SE is the better Grado partner.
I can tell you the WA6 SE does drive low impedance phones very well, as well as high impedance phones.
I had the WA3 OTL amp... and it struggled with low impedance phones - clipping upper mids, and bass - producing a rough, harsh sound.
The WA2 is a somewhat similar OTL design, but has greater power output, and I think some chokes, which Woo Audio reports is better able to drive low impedance phones. And... there have been some reports by users, that it does drive low impedance phones OK... but... not as well as high impedance phones.
But... after considering both... in the end... I bought the WA6 SE for the reasons Skylab described... and... I'm very pleased with it with both high and low impedance phones.