From a mechanical standpoint, all you would need to do is connect stereo analog cables from any sources you wanted to be able to listen to and then hook the second zone outputs to the spare channels of the 7125, which would then drive the two new speakers. The second zone is basically a small stereo analog pre-amp tucked inside the 990, though, so you wouldn't have the benefit of the 990's DAC or upsampling capabilities when using it. For that reason, I'd probably not use it as a two-channel music front-end unless you are doing so with a good stereo source. Would it be possible to upgrade your two main speakers and using them for both two-channel and multichannel listening?