It can certainly be done for all channels if desired but keep in mind tube replacement costs and the additional heat you will be adding to your room.
If you like to listen at full THX Reference Level you will need either powerful tube amps or efficient speakers. In my theater when I was running my front channels with a moddified Dynaco Stereo 70 for L/R and a MKIV for the center channel (around 30ish watts per channel) I had no problem at all playing back at +5dB over Reference level. But my front channels are about 104dB/w/m.
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