Never tried an SACD with the xr70 while in biamp mode as my Panny CP72 will not play them. However, with everything else like CDs and Movies, I seemed to get great sound out of all five of my speakers even when I had the fronts biamped.
One caveat, though. Right now I am running an xr45 and an xr25, digitally linked, biamping my front three speakers, and also using each of the rear sixth channels from the receivers to power a quasi 7.1 system. The sound is so much better then the xr70 could put out by itself--even in biamp mode. When running in stereo mode with the xr45/xr25, I am now getting much more bass out of my front two speakers--even my wife was wondering what I had done to get things to sound so much better. Not exactly sure why it would make so much difference, but I am happy with the outcome. The xr70 went right back to the store.