Subwoofer with XP-22 in unity gain

Keep the sub connected to AVR if you want it optimized for movies. 
Get another sub for music and connect it to line level RCA output. 

The danger in that is you switch inputs to another line level source and your volume stays at 89. 

Switching inputs on XP-22 maintains the previously set volume level.
If you have a digital source playing at that time you will pump that signal thru at 89 on volume scale when you switch to that input.
I hope that’s clear enough. What that means is a unity gain signal burst thru your entire system. What will be the outcome of this? Potentially fried component or speaker driver, including temporary damage to your underwear.

Use bypass for your AVR and don’t screw around. You will potentially learn a painful and expensive lesson otherwise. It is bound to happen. 

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. 

For now keep the sub connected to AVR.

Agreed.