I love my new Norh SE9 tube amp but have a number of basic questions, as a newbie tubie :-) 1. how will I know that the tubes need replacing ... does it just die, or kinda fizzle out slowly for a few days before hand? 2. How will I know exactly which tube to replace, or do I replace them all at once. 3. What's a good source of replacements? 4. Can tube be upgraded, i.e. are their better brands out there? Thanks in advance. Sure the answers will help others as well as me :-) Dave