All modern AVR's I've used have an auto-calibration setup routine, running a program such as Audyssey, that uses a microphone to "listen" to the room and automatically sets up the speaker configuration according the the levels the microphone detects.
I just mean the you put together the setup, as I described, set the volume control to your pre-determined level (such as top dead center, etc.) then run the auto-calibration routine. The auto-calibration routine will recognize that the front L&R speakers volume is "low" and it will increase the volume of the L&R speakers relative to the other speakers in your multi-channel setup. It will do this calibration automatically.
Only caveat is that I have never done this using a "passive pre-amp". I do not know if the auto-calibrate routine was enough variability to make up the volume difference. If that were the case, you would need to increase your "pre-determined volume control position" to account for that.
You could try it, using the passive setting, and you can also try it using the tube output, and see if there is any difference. Just remember to always run the auto-calibrate routine on your HT receiver if/when you decide to change from passive to active/tube.
Also, there may still be a volume difference when you switch from the input you use for your passive/HT source and other inputs/sources. I would suggest you try to find the correct "pre-determined" volume control position that somewhat matches the output of your other sources. You could do this by simply listening to music and switching between passive and tube. at some point of the volume control position you will find that the volume is the same for passive and tube "active". This would be approximately the "unity gain" position for the tube output and, if convenient for you, it would be a good position to use for your pre-determined position.
BTW, this all makes sense to me based on my assumptions regarding the Schiit Freya. I do not own one, and have never used one, so I could be off base. But logically, it all makes sense to me.