I need to replace the 12ax7 tube in my SWR 350x amp. There are numerous manufacturers and a few different types of 12ax7... 12ax7-s, 12ax7-c, 12ax7-a, etc. I am confused. Does anyone have an opinion as to the best choice? I play Jazz, Blues, and Classic Rock. PS; I recently bought an Avatar TB153 and I am really stoked with the sound. Thanks.