Tubes 1 and 2 seem to make the most difference in sound when swapping different tubes. I cant remember but I think the 3rd tube is used as a buffer where the first 2 deal with the sound. I dont remember any noticable changed when I was swapping out tubes. For a clone of the Ear 834p the t11 isnt half bad with the right tubes. It all comes down to what sounds best in your system.