I have the 661 and use the PLIIx matrix all the time. In the manual setup menu, there is a function that says "Ext surr" or something like that. You have to turn it on from there. I have a Harmony remote, and there is actually a discrete code that will do this automatically on power up. If you don't have that, you'll need to turn it on manually all the time.
Of course the above is for the 661, but I suspect it's the same. If you want specific button pushes to do it, there is a massive thread on AVS. I can try to dig it out if you need.
Pat