Any advantage having a 3V Dac output?

That 6x gain translates into 15db, which is lower than many other amps but pretty standard for the first watt approach.  I don't know enough about voltage input sensitivity to know for sure, but I suspect that value is also lower than many other amps too.

I believe that 15 dB gain (which is low -- most amps have 20+ dB of gain) means that the amp needs about 2 1/2 volts input for full power output. He should be fine with the 3 volt output of his DAC.

Any advantage having a 3V Dac output? I understand that a 3V output would be an advantage if one is using a passive preamplifier

Correct, you don't need any more voltage gain from an active preamp,
And you only even need 2v from the source as most power amps only need 1v to1.5v to get full wattage output {clipping), 3v will blow their heads off.

Cheers George