2 vs 8 tubes in preamp, sound difference?

Just for a different perspective...designs that are using 8 preamp tubes most likely are the more expensive preamps but no idea if that relates directly to sound quality. There are some good tube preamps with just two tubes. 

You have to take the ceteris paribus approach. Would the same basic design be improved by adding more tubes? It depends on how the design is implemented. Say you have a two tube preamp and it sounds good. Then you add another two tube voltage regulator which tightens up the bass and soundstage. Now double it for stereo and separation improves. Now replace the anode resistor with a tube constant current source to improve the fine details. Replace the diodes for a tube rectifier may add better dimensionality etc...

The Conrad-Johnson ET6 and ET7 sound fine w/just one 6922 tube.

But they also use MOSFET output buffers and solid state voltage regulation. Anyway, unless it has a phono stage, a preamp is a fairly simple device.

It's ALL about the design.