Let me reiterate what I said above: Watch out for the mix of headphones that you may encounter. Each pair of headphones that you add to the amp lowers the total impedance seen by the unit, and some headphones have lower impedance than others. Make sure that whatever unit you choose can handle the minimum impedance that you may present to it.
Some of these units will tell you what impedance they can handle, others won't. If there's no spec for it, be suspicious! The Rolls unit that I tried failed completely in this area. And the various Behringer units have very different characteristics. The HA4700 can handle a combined impedance as low as 8 ohms, which should get along with pretty much any combinations of headphones on the market, so the 4700 is a pretty safe choice.
The HA8000, on the other hand, will only go as low as 100 ohms. It would be quite easy to violate this limit with a few pairs of Sonys or other low impedance phones. And
the Amp800 doesn't even give a spec, so I'd expect the worst.
I don't know what the specs are on some of the other units on the market, but be sure you look into it before you buy. You could find a unit that seems fine until one of your clients brings along a pair of phones that you hadn't expected, and suddenly your nice clean headphone signal turns to mush.
That's exactly what happened to me with the Rolls unit. It sounded great until I actually went to use it in a session. So there I was, embarassed that I couldn't provide a clean headphone mix to four or five sets of phones. The result was NOT subtle.