I was waiting for
@gene or
@TheoN to give you a definitive answer but no show so far so I am going to simply quote jdsmoothie:
www.avsforum.com
"X8500H also has a "Custom" amp assign setting which allows you to map virtually any speaker binding post to any channel of output. For example, if you're using a 5ch amp to power LCR+Surrounds in the main zone, you can repurpose the unused FL/FR and SL/SR amps in the AVR to power Zone 2/3 speakers (the X6400H only has predefined templates for amp assign)."
If you go to post #2 and scroll down, there is a table, that I believe might have been copied from the Owner's manual.
So yes, you can do it by using the custom amp assign feature instead of the "pre-amp mode" that would disconnect the internal amps via switches
NJU72750.
The AVR-X8500H is the only AVR in the D+M line up that allows such disconnection between the pre-amp and power amp sections using the custom assign feature. The lower models such as the AVR-X3600H, through X6X00H, and SR6012 through SR7013 only allows you to do the same with the FL and FR channels.
By the way, even "pre-amp mode" will not turn off the internal amps off (this, is according to Gene iirc),it simply isolate the pre-amp from the power amp, so the power amps rail will still be "live". From the pre-amp stand point, it will still behave like the prepro AV8805, just no HDAM..and no balanced output.
Edit: I edited AVR-X3X00H to AVR-X3600H, because anything older than the 3600 in the 3000 series are capable of 7.1 only, therefore the FL/FR internal amps are not asssignable because it would be of no purpose anyway.