I ran across this article on Stereophile's web site from September of 2000 > "Tubes Do Something Special". The article goes on for 7 pages including follow-ups. The article contains a lot of good info on why tube amps tends to sound louder (and consequently cleaner) than their solid state counterparts of the same power level. Check it out at the link below ..
Tubes Do Something Special
Bob
Tubes Do Something Special
Bob