How about a bit of both? The pre-outs are generally always live. In most Denons all the amp channels still receive juice, even if using a pre-out, and are affected somewhat by that in measurements. I'm a bit rusty on which Denon does what with pre-amp assignment "purity" for the L/R or all channels, but IIRC mine (the 4520) only has the option to cut the speaker amps out entirely in "pre-amplifier" model. The 8500 can do it on any desired channel IIRC. Some only do it for L/R channel.
@PENG I think may know off the top of his head....or maybe we should start an updatable thread on various models from Denon and Marantz and their capabilities in this regard? The question is asked frequently enough it seems.
I certainly don't worry about it when I hook up an external amp, tho.
Not sure how the "B" speakers figure in this, tho, never used that option....how are you using it?