Its interesting that the criticism comes from the distorting sound of valves.

This is often thought of as their strong point...

IE Valves (when distorting) are thought to do so in a more musical manner than a distorting solid state sound. Its one reason why valves are used by audiophiles and guitarists.

As to valves/rock - Id disagree also. I think valves dont rock when you ask too much of them - i.e. powering difficult speakers with a not very powerful amp. True of solid state aswell, but high watts are cheaper with SS, hence why this mis-match demo happens so often IMHO... (hope that makes sense).

Unless you're saying that slight obfiscation of detail and added warmth-colouration are  distortion? Its a word that means different things to different people IMO

Theres no reason why a well setup valve system cant do rock, classical, jazz or anything else IMO.