Does the little Yamaha amp have gain controls for its channels? If so, you may need to tweak those a bit to even things out. What may also be happening is that splitting the pre-amp outputs reduces the signal, resulting in at least some of the imbalance you are hearing (although I think that's a secondary effect in this case).

If the Yamaha amp has gain control or you can add an external volume control (similar to the first device on this page ) between it and the 1070, you could consider using the tape output to drive the Yamaha. That would decouple the main system volume control from the other rooms. Since the tape output only gets analog inputs, you'd need to make analog connections from the sources you want to hear in the other room even though your main zone may not need them (the tape output would see the analog signal while the 1070 used the digital signal for your main zone). It's sort of a quirky way to create a second zone, but it might get the job done for your situation.