(Going direct to amp like this is the most transparent/dynamic sound you’ll get)
Their actual quote in the manual is this: "...due to a wide range of power amplifier designs, it is important the input sensitivity of the amplifier be at least 2 volts. This will assure the playback of of most music recordings are reproduced with adequate dynamic range."
Now to your question.
This is because the volume control is done in the digital domain, and "if" below say 75% of full there’s the risk of what’s called "Bit Stripping", this is a lowering of the resolution and dynamics from 16bit to 14 bit to 12bit the lower on the volume you go.
Wadia, Mark Levinson Bricasti and others, in some of their higher end models have a gain adjustment switching inside on the analog output buffer usually with pins about 6 different settings so you could set this to your "system gain", so then be able to use the digital volume control at or above 75%.
If you don’t have this and have to turn down the volume on the Wadia below 75%. Then you better off leaving it up full and using a passive preamp.