Try as I might, I just cannot hear a difference between the different tube types in the amp block. I've got pretty good ears, and I can hear the differences in most other subtle things, such as the new mic preamps in the cab block. I own a Deluxe Reverb and know first hand how different it sounds with new/old/different brands of tubes in it. I've also had a JCM800 combo in the past, too. So after a decade of playing on real tube amps, I've got a sense of how a few of the different tube types sound. However, I just can't hear any discernible difference in the Axe. Now, I have complete faith in FAS's programming and attention to nuance, so I'm sure this has to be a case of PEBCAK (Problem exists between chair and keyboard - or Axe). What am I doing wrong?