Subwoofer with XP-22 in unity gain

Please read your manual more carefully.  Input 5 is the Home Theatre (unity gain) pass thru function.  

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. 

@testpilot bruh, the last sentence of the first paragraph on page 6 of my manual says: 

You may also set any input to unity (0 dB) by selecting low gain and setting the volume at step 89.

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

@goleksiak, the 89db setting is just telling the user where on the volume scale is 0db, i.e. no gain or attenuation.  Also, if you set input 1 at 89, all other inputs by default will be set at 89. This could be a recipe for disaster.  Why not use the Home Theatre pass thru on input 5??

@testpilot My DAC only has balanced outputs & Input 5 is single-ended. 

@audphile1 if system volume is set to 89 (0db gain) then what is the harm in switching to another line level input (say a phono)? 

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. 

@audphile1 Ok. I'm new to all this which is why I'm asking... So I think I'm missing something about sources: Reading my MSB manual, it has Low (-6db) and High ("Standard"db) output settings. So sources can present to the preamp with different voltages....did not know that before. Is switching between sources with these differing input voltages what would cause the potential overload? (Or do preamps always expect to receive the same input voltages across sources)?

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.