Stefano
If you're just looking for a passive switcher box for powered monitors, check out this from Coleman Audio.
http://www.sweetwater.com/store/detail/LS3/
I used to have one and it worked great. I ended up getting the Mackie Big Knob because I needed the talkback function plus I could monitor many more sources (CD player, turn table, DAW mix, etc.)
Hope this helps.