There are scientific reasons tube based equipment will sound different to the human ear. First, a tube plays about 4 times louder per watt than solid state. A tube's harmonic distortion occurs on an even order as opposed to solid state devices where it occurs on an odd order. And yes, when tubes approach clipping, they are more "rolled off" than solid state equipment. Also remember that with tube equipment, one can use different brands and types of tubes which can affect sound quality. If you are considering a "single ended" tube amp as opposed to a "push-pull" type tube amp, you will want to consider a high efficiency speaker. Like with anything else, there are good and bad tube based equipment. Look for a well established manufacturer. There are a lot of "garage" made equipment that probably should be avoided.
