The HE-4XX should stay as a 'reference' headphone. - build your 'collection' around the gaps around this headphone.
I havn't heard the Sendy, but I'm not sure if it fills any gaps either.
Q701 could stay because it's a soundstage monster.
ATH-R70X could stay ... depends on how different it is to the Q701.. maybe they're too similar and one can go?
Fidelio X2HR - if you like it's madness, it's V-shaped and crazy enough to fill a gap.
Koss ESP/950 - can stay because electrostatics are fun, but depends how much you use them. Apparently they're not perfect and stats are the most cumbersome to use.
Generally, the rule is keep the ones you use the most.
In the end, you'll likely be swayed just as much by comfort and style as you might be by the sound presentation.
The eH-2200 got destroyed in storage (things shifted in the box it was in and crushed / twisted the headband) so it's done. Too bad, as they were one of my first sets years ago and I had a sentimental attachment, but no great loss in terms of sound quality or utility.
I once loved the ATH-M50 but (I guess) have outgrown them. They get almost no play and will probably be left at work in case I need headphones to listen to an online presentation or something.
The Mikros 90 is uncomfortable and I don't really care for them. It's a shame, because they look very nice, are reasonably compact, and are easy to drive. They would make a great portable option if they didn't crush my skull and sound mediocre. Apparently they have something of a cult following here. I'll probably be selling them soon.
The Sendy AIVA is more detailed / resolving / brighter than the HE-4XX, much more comfortable despite being heavier (/way/ better headband design) and better built all around. That said, it's a little bass shy compared with the HE-4XX or ATH-R70X which are both fairly flat (I tend to prefer a flat sound signature), so I'm not sure what it's "purpose" is.
I like the sound signature of the 4XX but find it less than comfortable so don't listen to it all that often. I actually have two pairs after RMAing and forgetting to return the first set, then realizing the issue was with the cable not the phones themselves. One has 2.5mm connectors at the earcups, the other 3.5mm. Now that I have balanced cables with both terminations I'm definitely getting rid of one, but I'm going to listen for differences for a few days first.
I don't listen to the Q701 all that much any more as they're in bad shape (the pivot point on one side is broken and barely holding together) but will probably keep them as a reference. They're worth little to nothing as it stands. I never considered the ATH-R70X might sound similar, but I guess that makes sense now that you bring it up. They don't have the soundstage of the Q701 and are a little less colored but over all, similar. The R70X are my most comfortable set right now and very neutral, so they'll be staying.
The X2HR are interesting for some things and leak a lot less sound than the ATH-R70X, making them more suitable for Skype with friends and that kind of thing. Yes, they're V-shaped and bass heavy, but much better behaved than the ATH-M50 over all, and "fun" sometimes. I like a "clinical" tone most of the time; these are the exception. They're also not /that/ "crazy" to my ears.
If I really like the Denon AH-D7200 I could see selling the X2HR, but based on reviews I'm concerned about comfort with the Denons, where I can wear the X2HR for hours without issue. Will see.
The Koss are their own thing and will stay if for no other reason than that.
So much for the great cull... Maybe the AIVA (though it hurts to even think about it -- they're so pretty), definitely the Mikros 90, maybe the M50 (though I'd have to replace pads and at that point what are they worth?).