At what point ( min or max value ) does av/preamp output voltage not make a difference?

I agree with Tom’s comments.

Also, while I haven’t researched the AVRs and Pre-pros you referred to, I suspect that the reason for their relatively low maximum outputs is that those outputs are generated internally by a DAC circuit. So as in the case of CD players those components are presumably designed to generate a maximum output, when the digital data is at its maximum possible value ("full scale"), that is modestly above the voltage that is required to drive most power amps to full power.

A preamp or line stage having an entirely analog signal path is a different story. In that case, to assure best performance the component should provide a substantial margin between its maximum output voltage capability and the highest output it would be called upon to generate under reasonable usage conditions. If that were done with a DAC-based circuit, though, the added margin would compromise usable dynamic range, noise performance, and low level resolution. And in many cases would force the listener to use the volume control at undesirably low settings.

The bottom line: While there may be some exceptions, in general that spec should be ignored IMO.

Regards,
-- Al

For a signal path in a preamp (or the preamp section of an AVR) which includes a digital-to-analog conversion, the maximum output spec should be (and in the majority of cases will be) higher than the voltage required to drive a power amp to full power.

For a signal path in the preamp that is entirely in the analog domain the specified maximum output voltage is usually irrelevant. (Although in theory, everything else being equal (and everything else is almost always not equal), a higher maximum output voltage might correlate with better preamp sonics to a greater degree than a lower maximum output voltage). But what is relevant in that situation is that the maximum output voltage of the source component, when increased by the gain of the preamp, should (at least preferably) be high enough to drive the power amp to its maximum power capability. While not being so high that the user is forced to set the volume control near the bottom of its range.

In most cases involving an analog signal path through a preamp, if the source component is a CD player or other digitally-based component and an active preamp is being used there will be no problem driving a power amp to full power. In many of those cases there will also be no problem driving a power amp to full power if a passive preamp (providing no gain) is used. That issue is likeliest to arise if a phono source is used, especially if it is used in conjunction with a passive preamp.

Regards,
-- Al