Single Sub Woofer for two separate systems?

You are encouraging feedback into the system that's not being used at the time.
I wouldn't do it.
You should unplug the one not in use.

Keep the ARC music gear unplugged until use and then unplug the HT receiver when using the SW with the ARC set-up?

or maybe keep the SW hooked to the ARC gear and only use the SW w/HT when watching movies or something worthwhile for bass. Don't really need the SW for watching regular TV or even sports, but for good action movies, yes.

You can do what you want if you have or can find an NHT X-2 XOVER.

You can.

I do - works perfect.

AVR sub out goes to the LFE input on the sub.

Integrated amp with HT Bypass with preouts goes to the line level inputs to the sub.

For movies, Int Amp set to HT input, preouts are disabled, and the AVR sub out drives the sub with the scary low LFE effects.

For music, AVR is off, and source components are directly connected to the int amp, sub gets full range audio and the xover is set to blend with my mains.

No feedback, no cable swapping.

Everything Erikt said, absolutely! EXCEPT you can't do what Erik says UNLESS your sub is equipped with L and R line level (2-channel) inputs and a separate (single) LFE (low frequency effects) input. I know the MartinLogan Depth and Descent are configured this way, and probably many others these days -- but check first!
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