Ah, I see what some of the benefits might be, no need for switch box as you say and you can control final volume of everything via the pre-amp whilst keeping all the Line-Ins at their maximum line level, and no twiddling of volume knobs on the back of the speakers either. Have I got that right? I suppose one drawback to that is that you're introducing another amplifier into the chain which can increase noise levels/distortion? And then there's the cost of the preamp?
EDIT: my current setup doesn't really need a preamp there considering the Xonar U7 and the mini DSP both output roughly the same voltage (1.1V & 0.9V), so volume twiddling on the back of the speakers shouldn't really be necessary when I have the U7 & miniDSP hooked up to the two inputs of the switch box I've ordered. The difference will occur when my E30 DAC arrives where it can output 2V...I may just use that for optical connection to the TV and it would output 2V to the miniDSP (max it can accept), and I'd use the volume control on the E30 to control TV volume. (At the moment I control TV volume via TV remote as I'm currently using the analog Line Out from the TV to miniDSP.). It might be sacrilege to use the E30 as TV/movie rather than music from my laptop (which is Xonar U7 currently), so I will also trial E30 DAC connected to my laptop to see if any upgrade in perceivable sound quality vs the Xonar U7 (I could volume match using UMIK microphone)...if it's not a clear difference then I'll use the E30 for the TV like I explained already, because maybe the weakest link currently is the DAC and Analog Line Out built into my TV (Sony KDL-42W653A).