The 4 V (just an approximate figure I think) without clipping Gene might have referred to in one of his videos or in his review on the AVR-X3600H, was about the maximum unclipped preamp output voltage when the AVR is used in preamp mode. In preamp mode, the pre outs are disconnected from the AVR's power amp inputs so the power amps won't be amplifying a signal and there won't be clipping (nothing to clip).
In the AVR-X3600H review, Gene stated:
Denon AVR-X3600H 9.2CH IMAX Enhanced AV Receiver Offers Best In Class Features | Audioholics
"With the AVR-X3600H configured to reroute the main front L/R internal amplifiers, the preamp is physically disconnected from the power amp. This allows the preamp stage full capability of delivering up to 4Vrms unclipped when connected to external amplification as can be seen in the following output vs distortion plot courtesy of Sound United Engineering."
If you don't use preamp mode, the preamp outputs will remain connected to the inputs of the AVR's own power amps. When the power amps are driven to higher than their rated output level, they naturally will clip and will affect the performance of the preamp outputs possibly via some forms of feedback, coupling and/or other ways, but that is just my understanding from 20,000 feet. For an official explanation you would have to ask D or M, but good luck to that, as you would likely have to contact their engineering team to get an answer.
Note: Even when no speakers are connected to the AVR amp's output (speaker binding posts), the power amps will still amplify the signal voltage so their output
voltage will clip when the input signal gets high enough.
Also note that as Gene explained in his review on the AVR-X3600H that does
not have preamp mode, you can still disconnect the pre outs to the front left and right channel power amps using the amp assign feature but again, in that case you can only do that to the front left and front right channels. You should be able to do the same, if necessary, with the AVR-X3700H.
Whether you are in preamp mode or not, the pre out (preamp output) voltage will depends on the input level, and also, obviously the volume control setting.