Ty’s suggestion about the Mackie Big Knob is a good one. You really need to have some control between your Mac and the Genelecs. Otherwise, you might be combing speaker cone out of your hair one day. I suggest you consider a small mixer (Mackie makes them, too). That way you can direct the audio from decks, cameras, I/O devices, etc. to the mixer inputs for capture and also monitor and control the output of FCP.
John
MacPro 8-core 2.8GHz 8 GB RAM OS 10.5.5 QT7.5.5 Kona 3 Dual Cinema 23 ATI Radeon HD 3870, 24″ TV-Logic Monitor
ATTO ExpressSAS R380 RAID Adapter, PDE Enclosure with 8-drive 6TB RAID 5
Final Cut Studio 2 (up to date)
Pro Tools HD w SYNC IO, Yamaha DM1000, Millennia Media HV-3C, Neumann U87, Schoeps Mk41 mics, Genelec Monitors, PrimaLT ISDN