Subwoofer with XP-22 in unity gain

That’s a nice setup! 
1 So your MSB DAC run into XP22 using XLR will be best. Say you are using Input One on Pass

2. Connect your AVR to Input 5 on XP22

3.  Connect the output from XP22 to your amp using either XLR or RCA outputs 

When you want to watch a movie thru your AVR, hit Pass Thru button on the Pass Labs remote. It will automatically go to Input 5 and bring the volume to 89db unity gain.
Say you want to switch back to MSB. Hit Pass Thru and volume will go to 00. Then hit Input 1 on remote and using volume button up bring it to your desired level. 
Bottom line, only use input 5 for home theater. Safest approach. 
Hope that helps. 

@audphile1 Thanks for this. As you can see, I'm trying to extract a hifi system from my current HT step-by-step while keeping them both usable. My end goal a separate hifi system (below). (Replace the AVR with AGD Vivace amps) and in this scenario, this is the way to utilize the subwoofer, right? (I'll get another sub dedicated for the HT)

For now keep the sub connected to AVR.

I messaged Pass Labs support and they confirmed that the configuration above is correct.

Agreed.