your post is confusing. I own the eversolo DMPA6 and Mac gear as well as a Lumin P1 - which can also be used as a Pre Dac.
The Eversolo can be used as a pre amp with a volume control and can be hooked up directly to an amplifier. Use the volume on the eversolo just like you would on any other pre amp. It can also be run into a pre amp that is attached to your amps. In this set up up you can either have the eversolo volume locked and set as "fixed" in which case you use your pre amp volume knob to control volume, or you can leave the eversolo volume as variable and you'd be able to change volume both on the eversolo and or the pre amp (no real purpose to having it set up this way though). There's no distortion with the eversolo or any DAC pre amp when used even up to 100%. All you'll notice is that your main volume setting on your preamp may be set a little lower than usual to make the same volume- again, with no distortion.