I do not believe the Preamp output on the Magni 2 U is just a Pass thru.The output is affected by the Gain Switch. Why Not Set the Volume level on the Speakers and use the Magni 2 U volume control for everything? I used Powered Monitors for some time I set the Volume level on the speakers and just used Magni 2U for volume control on everything. That way you never have to touch the speaker volume control once set. The Volume on the magni will bu used every time you use cans, then you have to reset it to use the speakers.