Does anything similar to the Motu Monitor 8 exist?

I'm also not necessarily opposed to just using a decent 8 channel interface with good routing, low latency, and wireless remote control to send to a separate HP amp, though that adds additional complexity and connections....

What interfaces beside the Motu stuff even offer wireless control?

What headphones have you used with the Monitor 8, impedance of the phones being an obvious question?

I commented about using the Behringer Powerplay system in response to your post on the Apollo X thread.

Let me just add here that using Extreme Isolation EX25's you can't really put the volume past 50% unless you are in fact a rock drummer :-) This is fed from power over LAN so quite a good volume headphone amp there. I have also run 2 headphones using a splitter where I had more musicians than mixers and they had no volume problems ... it was just a bit tricky to get them to agree on a mix. I think the overall price of the Powerplay is going to be higher as each mixer costs $250 so you would need to take that into account.

I am actually not sure I would want people to be doing their own mixes on their phone(if that is your idea) .. it's difficult enough to see what they are doing on the Powerplay mixers but at least I can diagnose issues by walking over and clicking through the channels to see the volumes they have set. In the case of the Motu .. you'd have to take their phone off them? Could be awkward if there is not a good vibe with the artist. I recently had a drummer tell me the click wasn't coming through at all .. he had the master volume way down and all the channels way up ... I did a quick re-set and he was happy straight away. I had only met him 2 hours before .. not sure I would have felt comfortable asking him to hand over his phone?

If so, I could just use one of the line outputs on the Motu instead and feed that to a standalone HP amp for the drummer if necessary.

I wish all these personal monitor systems had 24 or more ch!

Depends on the artists I suppose. Many would be intimidated/distracted by all that tweaking power when they are supposed to be focussing on their performance ... or maybe you are getting a better class of artist :-)