I've used the Rolls "more me" mixer..but in live applications only.... and they work, but are a tad hissy. ART has the same thing, I believe. You send a stereo mix, and the adjust themselves against that mix. Some have two inputs... a line and a mic. I agree with Mike, unless you're dealing with experienced musicians, do the monitor mix for them and a pile of extension cables are OK, I'd think. Don't know your setup though. If you want to get a tad more flexible, the Behringer P16...the Aviom substitute, seems to be holding up in live situations, and for about $750 should give you all the flexibility you need. About as flexible as you'll ever need and the lowest price. I'd stay away from some of the other brands, as you're already buying outdated stuff...if you can find it. But used, is an option.
In my setup, I use a [discontinued] Mackie mixer, that was geared specifically for headphone mixes. It has 4 inputs and a send, which the talent can adjust. I even use a reverb send on it, with a little unit the talent can adjust if they want as well. However, no one EVER touches it. I mean never because I think they're just more concerned with getting good takes and want to leave that stuff up to you. Otherwise a cheapie live sound, monitor mixer works too.