Single Sub Woofer for two separate systems?

You can do what you want. Just confirm that your sub will allow you to use 2 seperate systems. I have a pair of Velodyne DD10Plus that say that you can do that.

How about a Niles AXP-1 input selector? Take the subwoofer outputs from each system and plug them into an input on the Niles AXP-1. Then from the Niles AXP-1's output plugs connect that to your subwoofer. When you want HT or your other set-up just change inputs on the Niles. I use the Niles now and don't have any sound degradation issues. It is built quite well too for the $ IMO. You may or may not need some RCA F to 2 RCA Male Y adapters too. I had some laying around so I used them with my Niles. Good luck.

Thanks all. Unfortunately my sub (Carver) does not have a single LFE input. Instructions say to use the Left Line Level input for a single cable to the AVR.

The sub does have controls for a xover, phase, and sound level.

It also has Speaker Level inputs where I could double wire the sub with my front audio speakers. Did this before and seem to work fine.

Currently, I have the sub hooked to the music amp/preamp by way of splitter off the Main Pre outputs. I believe that is the preferred method.

I also have a dedicated electric line with hospital plug so I could separate the two systems from each other. Double wire the sub from the Speaker Level inputs to the audio amp speaker outs, keep the single LFE cable hooked to the sub's Left Line In from the AVR, and just make sure one system is off?

James, if I had the Carver sub to work with, then I'd work with it, meaning: don't ask it to do something it wasn't meant to do, and in the process, degrade the performance of your system in it's other (non-HT) modes.

Just enjoy the audio (in all modes, both 2-channel music and AV HT) as a two channel system with a subwoofer. The LFE functions don't provide that much xtra low frequency versatility, especially since the sub already offers x-over, phase, and level control right at the sub -- set it once and forget it.

It sounds like you don't even have 2 sets of main outputs on your preamp, and splitting the preamp's output already introduces a small but noticeable amount of sonic degradation.

Until you can upgrade your preamp (to one with at least two sets of main outputs) and your sub (to one with separate LFE and 2-channel inputs) and in the meantime if you just must do SOMETHING ;--) I'd go with Jedinite's suggestion and use a Niles or other input selector switch, which will make it easy to select between the two modes of operation within the capabilities/limits of your current equipment.
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There are two outputs on the pre, one main and one tape but I don't believe the tape will work.

Not really concerned about using the splitter on the main out. If there is a sound degradation from previous set-ups, I don't notice.

Thanks, I may look into the switch.