I've always been a little mystified by those who have multiple 'good' headphones and switch between them.
I come from the speaker world where you try and have one great model. Because it makes no sense to buy something that isn't as good as that one.
There is a well-accepted science theory that the closer you get to perfection the more similar things are.
Amplifiers are an excellent example.
Likewise, as Amir's reviews show, good speakers tend to measure and sound similar, with the main difference being bass extension and volume.
The fact that these TOTL headphones seem to sound so different you need multiples for different genres/moods indicates either that none of them are approaching perfection, or that no-one can agree what perfection is.
Hence, I guess, the attraction of the Harman Curve.
However, I'm dubious about these differences.
Due to changes in where I live I can't listen to my (very good IMHO) speakers as much as I'd like to so I've been investigating headphones recently. I've owned HD580s for a very long time.
I've been using EQ on my speakers for many years, and find it worthwhile but very tricky due to room effects.
EQing headphones, however, is trivial in comparison.
I've tried out HD560s, HD6xx, Hifiman Deva, Audio Technica R70x, Focal Clear. And a bunch of IEMs ranging from Shure SE215s to trendy Moondrops/Fiios to even trendier 7Hz Timeless.
All well-regarded.
But once EQed, they all sounded far more alike than similarly well-regarded speakers have. The one I chose to keep (ATH-R70x) sounds great on my usual classical and jazz, but also great on my occasional rock and reggae. Just as a good speaker would.
I'd expect something like the HD800 with EQ to be even better. And if it is I don't understand why you'd keep an inferior model. Unless you needed the isolation of closed headphones, or preferred the presentation of closed phones.
In which why would you need the open model?
A good headphone (or speaker) is an accurate one. And thus it should be good on everything.
Which is long-winded way of saying to the OP that if he has a budget he should buy the best one he can afford and hear the difference from.
If he doesn't have a budget, go wild.
I often feel that because (comparatively) good headphones and IEMs are much cheaper than good speakers (a $100 IEM like a Moondrop is vastly closer to the state of the art than a $100 speaker) people buy just for the heck of it.
Nothing wrong with that, of course.
A little bit of my history with Headphones. When I started the hobby, I got some entry level "audiophile" headphones like the Beyerdynamic DT880 and Sennheiser 58x. I've always used a little bit of EQ on my headphones, but it was just a slight bass boost, or reduction in treble peaks. I mainly watched youtubers reviewing headphones and was impressed by the "audiophile lingo" they used, like transparent, fast, revealing, slam, punch, etc. I still don't know what half of these words really mean when it comes to audio...
Anyway, since I didn't use much EQ, when I compared one headphone against the other, there were very significant differences in terms of frequency response. Then I figured out that, after I started buying more and more expensive headphones, like the Focal Clear for example, I'd miss the bass of my old Audio Technica ATH-M50x, that costs a fraction of the price. In other words, If you don't use EQ, there's a big chance you'll end up owning more than one headphone and you'll be switching between them based on the music you're listening to. That's what happened to me at least.
Once I found this forum, I started testing all of my cans EQed to Harman and I was shocked to realize my cans sounded very similar with the proper EQ. So what I thought was "clear treble" was probably just a spike in the 8k region. Still, even after all my headphones are EQed to Harman, I can still hear some differences in quality and, of course, the soundstage changes from one headphone to another, which may be a justification to own at least 2 or 3 cans if you're in the hobby.
Also, at least in my case, there's a collector's thing going on. I like owning several headphones and play around with them, test one against the other, with EQ, without EQ, etc.
Objectively speaking, you're correct, I should stick to the best headphone I have and sell the rest. But what's the fun in that?