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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