I'll cut and paste the same answer i gave someone else today if that's ok?!
I would suggest a passive volume attenuator if you are going into powered monitors. You can do it cheaply with an SM Patch Pro type for about £100
http://www.dv247.com/studio-equipmen...enuator--51232
or if you have the money, an SPL 2 control (£300 ish) or similar (They do the MTC 2381 with talkback facilities etc for about £500)
Stay away from active boxes like the Mackie Big Knob imho, they will colour the sound too much.
No need for a mixer necessarily.. I mix in the box most of the time and these will allow you to do that and still send headphone mixes etc.
You might consider the new Omni interface..it has volume control and a cue mixer built in etc. I use a 192 which sounds great but i would probably buy an Omni if I was buying now for all the convenience it offers. Looks good!