I went to Hunt Amplification and met with Lowell to discuss changing my tubes to NOS tubes. I talked with Lowell for a bit about my needs and he suggested and tried several tubes before going with what sounded best. He also suggested some further modifications. I took that advice with the caveat that I love my amp so was a bit nervous about making changes. He told me he would reverse the minor changes if I did not like the sound. I did like the changes. My AC30 sounds even better. Also, I was able to keep changed components so if I ever wanted to sell the amp it could be stock again. I am a very happy customer. Lowell does what he says he is going to do when it comes to customer satisfaction. Top notch.