+1 on
Wavebourn's observation. If you read all the opinions about various tubes you will find almost no useful consitent trends about what sounds good. Almost any sonic change in tube swapping, even amongst tubes of the same designation, and absolutley amongst tubes of different design, is due to different operating conditions of the particular circuit involved. Tubes have a reasonably complex and messy transfer function, and one that is highly dependant upon the operating conditions. Even small changes in bias, anode voltage, output impedance etc etc can cause large changes to the transfer function. If you were to swap tubes, and then tweak the operating parameters to the point where the two tubes being compared were operating with similar transfer functions the sonic differences would mostly vanish. A good amp designer voices the amp to the tubes he uses, and voices the tubes. The parameters are subtle, and at first glance, two circuits may look essentially identical, but only a couple of component value changes can radically change the sonic character.