Gnsc, 4.5V is the absolute **maximum** that a DAC should produce for 0VU on the CD. We've seen players that make as much as 15 Volts; even 7 Volts is -no other way to put it- retarded.
Amps that need over 3 volts are unusual to say the least, consequently going over 3 volts means that there will be significant reduction in resolution if the internal digital volume control is used. If you are running direct, there is simply no need to be able to do anything more than barely clip the amp at full volume- if maximum resolution is your goal (which I hope is the case with any serious digital system) then this should be regarded as a cast-in-concrete rule.
If this rule is not followed, then it will be easy to get better performance by using a good quality preamp as others have observed, because the volume control in *almost* any preamp made is better than *any* digital volume control made.
For a digital manufacturer to put the time and development into a high quality control just to get it shot down by this rather simple issue makes absolutely no sense to me, but things play out this way more often than not!