how to disable volume control on DAC/Pre amp

While you have both hooked together, and turned off, have the Pre`s volume turned all the way down and the DAC`s volume turned all the way up. Your DAC`s volume is not attenuating the signal while the Pre`s volume is. 

Then, turn both on, and operate the LA-4`s volume control as you would normally do.

Let me add: While operating as described, and you have too much gain, then, incremently turn down the DAC`s volume to further attenuate the signal.

Is the mishap you are trying to avoid accidentally turning them both up at once and getting too loud?

I run with 2 volume controls in series all the time and always have one turned up all the way and control with the other one.  there is no increased risk in doing this.

Now if you want to bypass the volume control resistor to improve signal quality, that probably can't be done in a solid state DAC like you could on a point to point wired tube amp.  and you won't have an old fashiioned sliding coil resistor in your DAC causing signal degradation.

Jerry