You should have gone for the 144. It has independent knobs for the headphone and line outs.
Since you didn't do that, now you are going to have to figure out some type of external control for the stereo RCA out signal to your speakers.
Use your imagination.
Main: Win 7 64 | Cubase 7.5.30 64 | WaveLab Elements 8.0.3 64 | i7-950 3.1 12GB | Focusrite PRO 40
Mobile: Win 7 64 | Cubase Elements 7.0.7 64 | Core2Duo 2.4 8GB | Tascam US-144MKII