Thank you for bring this up, unfortunately no matter how many times we post on this topic, similar questions due to people's misconception or confusing about the related specs keep popping up and people end up worry for nothng.
I'll just do a quick recap as follow:
1) Denon+Marantz AVRs do not turn off the internal amps.
2) D+M AVRs, from the AVR-X3700H and up, newer models included, have a preamp mode.
2a) In preamp mode, D+M AVRs will enable ECO mode automatically (not in the specs, but reported by some reliable sources, including Gene iirc..) so the unit will in fact runner cooler as the bias current for the power amp will be much reduced in preamp mode.
3) In preamp mode, the AVR will disconnect the pre amp output (so called pre-outs) to the power amp inputs.
4) With the internal power amp inputs disconnected from the pre outs, the power amps are still powered, but since there are no signal for them to amplify, they will never clip.
5) Because the power amps will never clip in preamp mode, there will be no distortions due to clipping fed back to the preamp side, as a result the pre outs will have lower distortions at higher output voltage, right up to its clipping point, that according to Gene (Audioholics) and Amir (ASR)'s measurements. That would be in the range of 3 to a little over 4 Volts.
6) The AVR-X3600H, AVR-X4400H, 4500H (not sure about the 4300), SR6011, SR7010 through 7013 do not have preamp mode, but you can use the amp assign function to have the pre outs disconnect from the power amp inputs, just like the preamp mode, but you can only do it to the front left and front right channels. Gene was the first only discovered this and shared with Audioholics, in his review of the AVR-X3600H:
Denon AVR-X3600H 9.2CH IMAX Enhanced AV Receiver Offers Best In Class Features | Audioholics
In that review (actually preview) Gene also commented that:
7) So basically, in 6) above you can only disconnect the FL and FR internal power amp channels whereas in 2) you can do that, or if you use preamp mode, all internal power amp channels will be disconnected from the pre outs.
8)
It is important to understand that even if the internal power amps are not disconnected from the pre outs, it is not true that the pre outs will be clipping at 1.2, 1.4 or even 2, 3 volts, the pre outs can still go up to 3.5 to 4.0 V or a little higher. It just mean that from about 1.4 to 1.45 V, it will generally start being affected by the clipped power amp output distortions fed back to the preamp section, thereby causing the pre out distortions to rise rather quickly. That is shown in ASR's measurements, at 2 V, the preout distortion would be about -75 dB, that is 0.018%THD+N, still not below the threshold of audibility. At voltages higher than 2 V, say at 3 to 3.5 V, THD+N will have increased further but not by much, because the distortion from the power amp clipping will not affect the pre outs proportionally, at some point it would taper off.
I have spend a lot of time posting some or all of the above on AH, I wish there is a quick way for people to find the answers either from my, or others including Gene posts.
