http://www.headphone.com/pdfs/HomeAmpManual.pdf says nothing about the output power.
Well, guess I'd go for the HE-500 as you have what must be considered a decent amp. I could see someone be more happy with HE-400 and a beefier amp on the same budget depending on taste, though.. But in the long run I think you'll be happy that you bought the HE-500.
IMO the HE-500 has much better sound profile than the HE-400's which I think has the kind that an amp can't fix. Of course I just pissed off the HE-400 contingent.
Yeah, but again I could definitely see someone liking the HE-400 combo more. Is about taste. If I only listened to EDM, I'd see no reason to go for the HE-500 over HE-400 for example. Or one might just like the signature HE-400 has. Not that I do