Lucas - I think you inadvertently bumped a 3 year old thread, but I'm still alive and kickin and happy to chime in.
I liked my WA3 quite a bit with hd650's several years back, but it lacks the dynamics and detail (and more sophisticated circuitry) of similarly priced otl amps that are now available. The bottlehead crack is now the budget tube amp to beat in my opinion. It is a totally beginner friendly diy kit (my first such project in fact), and is also frequently available on the FS forum or through builders that work with bottlehead. It is cheaper than either of the woos, and to my ears, far outperforms them with any of the 300ohm Sennheisers or 250 ohm+ Beyers. The popular speedball upgrade loads the crack's tubes with constant current which avoids voltage sag and provides a tighter, cleaner sound than what you get from the woos (arguably, even among their more expensive models). A transformer coupled amp, like the WA6, would still be a better choice with low impedance headphones though. OTL's and low impedance phones are a bad match.
With respect to hd800's, I would recommend against the WA3 based on my time with both amps. WA3 (and its more powerful, more highly resolved sibling, WA2) have a full and pleasing sound that is very nice with less agile headphones (e.g. hd650's), but they lack the detail and dynamics to show off what hd800's can really do (all my opinion of course). Others in the hd800 thread still recommend the WA2, but it costs 2x what a crack does and doesn't have a constant current supply for its tubes. I spent quite a while with both the WA3 and crack, and even before installing my speedball, the crack was an obvious favorite for me. It is an awesome hd650 amp, a great budget pairing with hd800's and a super fun way to learn a little diy. Depending on your price range, you might also take a look at the recently released bottlehead mainline - which is an absolute totl match for hd800's.
