Single Sub Woofer for two separate systems?

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.

A tape output is not the same as a main output. It does not go thru the bal or vol controls.

The Niles SAS-1 switcher will do the job, you can use one pair speaker with one sub for Ht and 2channel system.

Thanks Imhififan, I'll look into the Niles switch.
Sounds like a good way to go.