Decay and slam or impact are different. Decay is after the note. Impact / slam is at the beginning of the note.. so the speak. If I understood you correctly.
So, objectively I would say the HD800 is technically very close to the sr-009, but most say the stax is in a different league. I've never heard it and if I was just going via objective information I would say it is only slightly better. It just shows how very small nuances in measurements can amount to big subjective claims. It seems this is going on here to me. (I've never listened to the HE500 buy I've always had my eye on it) . Great banter though!
Sorry, but objective stuff lol. is base of data to support it, not by what you say.