I had a chance to try it out and it seems to do the trick. I used a RadioShack part - will probably replace it with something nicer eventually however at this length there is hardly any technical justification. I actually had to drop the volume in both the sides and the rears for the 7.1 input setup by about 6dB (twice) to compensate for the added speaker. Splitting the signal is probably not affecting anything - input impedance of the preamps is quite high and does not load the line outputs from the CD/DVD player. On the contrary adding additional amplified channel boosts the signal roughly twice (6dB).
I listened to
Hell Freezes Over and the sound certainly opens up. It feels much airier so to say. One thing that was annoying me on this open air concert DTS CD was that the applause was coming slightly from the front (it is mixed into all 5 channels and my sides are slightly in front of the seating front row). Now with the backs working this sounds as intended - from all around you. Makes you look around and check - who is clapping so loud next to you between the tracks. When instruments are mixed hard into the surround channels (like percussion in the right side in Hotel California) they now sound directly from the place in the wall where there is nothing (between the side and the back speaker), but the direction is kept tight. Very enjoyable and lets you almost forget where the speakers actually are. Here is how it all looks now with the QS8's in the sides and the rear:
Back of the room.
