The volume bypass is to allow integration with a home theater set up. Depending on the DAC, I suspect you'll not improve anything by using the DAC's volume control.
Usually when a preamp has bypass it goes to Unity Gain. The point where the input signal will equal the output signal.
You pose an interesting question. Generally, when we listen to music the preamp acts as an attenuater to keep the power amp from playing at full power. We attenuate the volume to a comfortable listening level.
If the Gamut was at unity gain, then the DAC would have to attenuate. So if the signal in equals the signal out without any attenuation the sound level would be very loud.
So if I understand what you are asking you wouldn't lose the Gamut gain, it would be a fixed point called unity gain which would be at a high level in order to equal the input and output signals.
I hope I explained that without it being confusing. For example, my system, I use an input at unity gain so my power amp and speakers can be the front left/right for my home theater set up. When my preamp goes into unity gain the volume read out says 75dB, I don't think I've ever played my 2-channel set up that loud. When my stereo preamp in at unity gain, I use my HT preamp for volume duties.
Your DAC will be doing the same thing as my HT preamp in my example.