The ultimate receiver can't be used as a separate power amp in the traditional way because there are no external jumpers to separate the pre and power amp sections. If you use an external preamp into the 8 channel input of the sunfire, you are basically connecting a preamp into another preamp.
What MAY work is this: (and we should get conformation from others first)
Connect the external preamp to the sunfire 8 channel input as desired. Turn the Sunfire volume to absolute minimum. Turn the external preamp volume to maybe half volume. Play some music and very slowly turn the Sunfire volume up until you hit the volume you want. Then, leave the Sunfire volume at that setting and don't touch it. Use the external preamp volume control to do all volume adjustments. That MAY work.
I have an URll and I just use the TV to switch HDMI. The TV has 4 or 5 inputs. Then I use the digital/analog inputs of the Sunfire for sound. Its a little more tedious but it works for me.