I couldn't do a direct comparison, but I personally think there are less differences between AD and HE-500 than there are between MD and HE-400.
And personally, I'd grab an AD over the HE-500 any day. No joke.
Even without a direct comparison, the AD was just so comfortable, so good, and so unique that it deserves that much attention away from other headphones in the same class or price range.
It's not like I'm trying to upsell the AD for Dan, by the way. It's just... I am truly wowed by the Alpha Dogs. There are just some things words can't express. If I was reading about the headphone, I'd probably be pondering the same question like you. But I saw the Alpha Dog in person, and I have seen the HE-500 in meet conditions at least twice. Alpha Dog is love at first sight. HE-500 is just... a good headphone, but not "wow, I must have this".