Here's a link to a 1996 undergraduate thesis from the School of Electrical Engineering, University of New South Wales titled "Analysis of audio amplifier distortion using DSP 0 emulation of vacuum tube type distortion"
http://www.michaelneale.net/MichaelNealeThesis.pdfHaven't read it entirely but from what I browsed, the author seems to imply that tubes sound cool!
Happy reading
romymartinez