second, they all add coloration to the sound so you are not getting true high fidelity because even the best do not have a flat response over the entire audio spectrum, especially when compared to modern day solid state amplifiers. Generally there is a roll off of the high end that some people like but one shouldn't fool oneself into calling it high fidelity. Second, the quality of tubes made today are not up to what they were when tubes were it.
Tubes are actually more linear than solid state devices, try using a transistor without any negative feedback. The only reason the high end frequencies can be rolled off in some tube equipment is the output transformer. You don't have to use output transformers in tube amps.