Do Audiophiles usually keep the gain of the digital source at around 80%?

Most of my DACs have had a fixed analog out.    The few that did were set to fixed or vol was at 0 dB

I'll always use a preamp so I am likely to buy a DAC with no preamp function.  I have had a few with volume,  the NAD M51 DAC had one , it was actually pretty good.  

The answer for me, it depends on the system synergy and the Voltage drive of the pre-amp to power-amp. Using my LTA preamp, when connected to the XA-25 driving the Falcon Gold Badge, it's normally at 50%, but when I have the X150.5 with a voltage driver pre, that sets the volume to 40%. 

BUT I have never noticed the sound getting "thin" as the volume setting is increased. To me it EVERYTHING gets louder until my ears cannot take it anymore.

As expected, if the recording is bad (for all the wrong reasons) YMMV.

but a good recording, be it streaming or Vinyl, the settings above is my DeFacto standard,

sometimes, I would pop it up another 20 to 30 DB to just my blood pressure up :)

In most instances the DAC has sufficient gain and sufficiently low impedance to drive the power amp through a pot. InnuOS has the server recommended at 100% which has nothing to do with the question. It is the DAC’s output section that matters. BTW: absent serious but rare impedance mismatches all that a pre adds is more distortion/ colouring.

BTW: absent serious but rare impedance mismatches all that a pre adds is more distortion/ colouring.

It sounds like your experience may be limited to budget preamps. I would advise auditioning high-end units. An excellent, thoughtfully chosen pre that synergizes well with your system has infinitely more impact on sound quality than $20,000 worth of tweaks.

I recently read in an old Stereophile magazine that output voltages over 4v are apt to overload preamps, yet 4.2v seems to be a current standard output for DACs.  Why are they doing that?  Is it just to boost their S/N ratio specs?