There is definitely a difference between "tube sound" and solid state. Caused by the differences in harmonics generated when the amplifiers are driven into clipping during program peaks. Differences in slew rate and damping factor also contribute. But this isn't something you are going to duplicate by smearing overpriced nail polish on a D/A converter chip. [Linked Image]

A pretty good article on the tube audio scene appeared in IEEE spectrum a while back:
http://www.spectrum.ieee.org/oct05/1640