With tube gear, there is nothing we know today that was not known 70 years ago. It comes down to quality of the parts and the skill of the builder in voicing the component. Foremost in importance is the output transformer, followed by the coupling cap and then all the rest of the components. Depending on the kind of sound one seeks, it may not be the case that modern components are better. Whether it is the case of cost cutting or a lost art, many of the best sounding transformers are vintage. Likewise, many prefer the sound of older caps even though they don’t measure as well (not as tight tolerances, higher ESR, slower slew rates, etc.). Even resistors matter and I know builders who prefer old carbon composition resistors over the latest offerings with ultra tight tolerances.