After doing some listening to both the WS99 and HE400, I'm not sure that someone who likes the HE400 would be satisfied with the WS99…except me. For starters, the bass is thicker on the WS99 and although it's not a slow bass, sounds rather slow compared and mid-bass heavy. As for the rest of the spectrum, the HE400's space around the instruments are just down right superior. It's like listening to music live compared to sitting in your room.
Then you gotta look into driving the HE400. The WS99's are one of the easiest headphones to drive and you will need a decent amp to drive the HE400. I've listened to many of the popular portable headphones here at Head-Fi and the WS99 rank near the top, if not at the top. I absolutely love this headphone and this is coming from a guy that loves Hifiman stuff (many seem to be bass anemic but I find them almost perfect on some).
So, if you're not super critical about your gear and enjoy music in all different flavors (as long as it's tuned well), then you will love the HE400 and the WS99.