It's all about the DAC's. The only way it makes sense to use the digital out on the PS3 is if the DAC's inside a preamp are better than the DAC's in the PS3. If the DAC's in the preamp are better then you want to use an optical cable. If you get an optical cable you want to get one that uses quartz fibers for light transmission.
Personally I recommend doing what I did which is to get a surround processor but keep your tube gear. That way you can have surround sound for games and movies and still have the tube sound when you do two channel. Besides the preamp you will need another 3 or 5 channel amp depending on how many speakers you want to run.