Currently the output voltage maxes out at 0.78 Vrms, or around -4.4 dB from what you want.
That said, it should be higher. The DACs actually output 1.1 Vrms (-1.4 dB from what you want), but we had to limit it for a limitation of the old amplified units. That limitation has since been removed in hardware, but for backwards compatibility we never updated the software config. We really are past due on a split in the config so that we can take advantage of the full voltage range on newer amplified units and streamers. I’ve added an issue here:
If you wanted to get your hands dirty, you could manually edit /etc/asound.conf so that pcm.ch0_softvol.max_dB is -4.1 and pcm.ch[1-3].ttable.X.Y are all 1.0 instead of 0.64. I haven’t tested it here but that should get you equalized volume across the DACs and maxed out at 1.1 Vrms. Note that I’m referring to the lastest asound.conf that was just updated in 0.4.3, and this config file will be overwritten the next time you update.