Why not run it though yer pre?
Connecting the speakers directly to the plasma will definaly yield crappy results
I guess you could always get a speaker selector and just use it backwards. Connect the speakers to the input on the selector and connect the plasma to the B input and the pre to the A Input.
As to how much it would affect performance? Well, yer speakers wont sound too good with the plasma's little amps, and when listening to music you just have another piece of gear in the way of the signal, could definatly degrade sound quality there as well
If you just connect the plasma and tha amp both to the speaker, yer definatly gonna hurt yer plasma's amps, and quite possible yer main amp as well.
Think of it like this, if yer runnin 100WPC to the speaker, and there is another set of wires going out to the PLASMA, yer basically feeding 100W of power to the OUTPUTs on the plasma's crappy little amp, and will probably destroy it.
Why can you not just run analog connects from the plasma to the pre? It wouldnt damage anything, you would have WAY better sonics all the way around, and everything will once again be Zen.
Honestly, if it were me, i would run analog outs from the plasma to the Pre. I wouldnt even try any other method, cause plasma's arent cheap, neither are good speakers. it is not worth the risk.
Run the analogs or dont try to integrate it at all in my opinion.
If it is worth doing, it is worth doing right, and with the dollar value of this equipment, i wouldnt get too squirrley and try to rig it, or you might find yerself with a bunch of broken gear