I don't know anything about the A8A except what's in the manual. I can tell you with the Denon, you can disconnect the individual channel pairs and the center channel so that the power amp's distortion won't influence the quality of the pre outs. "shut down the amp channels" is ambiguous, in the Denon preamp mode, it disconnects the internal power amps to the pre out but the power amps would still be "powered", just not getting the input signal from the preamp.
The advantage of the preamp mode is, it allows the preamp outputs to have lower distortions at higher output levels, and it saves a little energy as well as letting the unit to run a little cooler.
The advantage of not using the preamp mode but just disconnect the channels that you will be using external amps, is so that you can use some the internal amps to drive speakers that don't need as much juice. If you use preamp mode that you cannot use any of the power internal amp channels.