You could definitely do it, but putting your signal into a lower powered power amp also means that it will have different tubes, therefore changing the tone quite significantly.
And I would also assume that you would not be able to get the volume down a whole lot lower. Remember: twice the wattage only equals another 3dB of volume (Or is it clean headroom? I can't remember).
My 20 watter can get pretty darn loud, so I definitely would not be trying to use the poweramp of it for the preamp of my 50 watt, if the intent is to achieve lower volume.