Adding a valve can only IMO add distortion, and that is exactly what it does, however it is a distortion that sounds good to many, largely even harmonic. However I personally would concure with the likes of Peter Walker and his statement that an amplifier should be 'a length of wire with gain', therefore to deliberatly add distortion is realy a HiFi no no. If anyone can convince me that adding distortion adds refinement or detail then maybe I am barking up the wrong tree, and should take up knitting or pot painting for a hobby.
John...