Does your TV have line-level outputs? Most do. You can connect these to an unused input on your reamp/amp.
No. I would not hook up a speaker to two different amps (plasma counts as an amp in this case), I believe this causes problems for the amp not in use. At least not without a switch box of some sort. I think it's because the output stage of the running amp will try to drive the output stage of the other amp (instead of the speakers) since the output impedence is so much less that the load the speakers present. I'm sure someone will correct me if I am wrong.
Which begs the question Kal is asking, why not just use the stereo? When i had a "teevee" in my rig, I still burned ($$$) the tubes in my preamp and amp. You do what you gotta do! What's the big deal?