I'm so happy.
Got the setup working,as described, using the multi channel phono line out from my Onkyo hdmi-capable receiver to the far superior amp section on the Sunfire UR-II via its 8ch analog input section.
The 8ch. Input is used, and I have tuned the sound output levels so that the old receiver powering the side channels only is in line with the sunfire driving the rest of the speakers- via my trusty radio shack sound level db meter.
I have even tuned the setup so that signal level out to the sunfire is offset -10to -12db, set in the hdmi receiver, so as to gain the maximum possible output level wise headroom overall.
When the Onkyo only drives the side channels, it might be able to follow the Sunfire driving the other speakers via 6 amp sections. (6.1Setup, biamping the main L + R stereo speakers using 4 amp channels).
The channels run from the onkyo receiver is sent as phono signal at a relative lower level, and the sunfire boosts the real output via its own volume.
Thus I gain more headroom by using the superior amp power from Sunfire to the maximum potential...EEspecially driving a direct stereo signal I hope to gain max level and quality.
(And the onkyo I hope can follow in SS, driving only two medium B&W 600-series speakers..)
(The more the signal can be amplified, starting low at phono line level, the more space to the top level - output wise, via 2x200w. pr side for the main speakers. And I think the sunfire far surpasses my onkyo, no matter the more than10db signal level difference
This way I hope to get a maximum sound pressure level in stereo .. that's probably as high as I dare drive the old trusty B&Ws.
Thanks for the help guys.
Johannes