I'll try to stay mostly on topic here...
Are you kidding ? For $250 the choice is obvious, HD 800 S, I also prefer musicality but we're talking about $800 versus $1500, but if it's musicality he wants, an equalizer or maybe a tube amp can get in the way.
I suggested the AFO because
@gken92 said they liked a lot of modern pop music. The extra mid-bass and more forgiving sound will suit that genre better than the HD800S. Most modern music, especially pop music, is poorly mastered and has audible distortion when using the HD800S. The AFO hides it a little better. The extra mid-bass of the AFO over the HD800S will make the music sound more engaging and exciting.
I suppose to argue for your side, $1050 is a good price for a good condition HD800S. There would be little loss (besides time) to the buyer if they didn't like it and decided to sell it for the AFO.
I'm not saying the HD800S is the worse headphone. It's technically much better than the AFO and I listen to it more than any other headphone when I'm at home. But it does need a good amp and good source material to get the most out of it.
I don't usually suggest an equalizer because I hate how inconvenient it is. I have to make sure it's on and set to the correct headphone and have different settings for different DACs and amps and change it when switching headphones, etc. As for the tube amp, I agree. I like tubing the HD800S. My only reservation with tube amps is that the sound varies wildly depending on the amp and then even the tubes. I can get substantially different sounds from my LD MKIII depending on what tubes I use. My favorite tube amp pairing that I've heard with the HD800S is the Eddie Current Zana Deux.
I suppose on a slightly unrelated topic, my AFC is my favorite headphone for music when paired with the iFi micro iDSD BL. I would usually suggest it over the AFO for someone who has heard neither Aeon.
You don't need a nuclear plant for the HD800, you just need a decent amp with with good tonality. Something like the Asgard2 I found good enough to use for my HD800 when I had it.
Can't speak for the HD800S, but the HD800 is a tremendous headphone once you put some damping on it. It's the most impressive headphone I've ever had the pleasure of owning and/or listening to. The reasons why you would want an Aeon open over the HD800 is if you wanted much less treble and forgiving of a sound and/or much more mid-bass or upper bass bloom.
I agree you definitely don't need the "nuclear power plant" but good tonality is a must. I found that I like the stock HD800S sound. I haven't heard the original HD800 so I can't really compare them.
The more forgiving sound and the extra mid-bass presence is exactly why I suggested it over the HD800S.