The Octal Preamp...Magical? If so, why not more Common? Is the “key feature” tactile response? Has anybody here built an Octal Preamp Tweed Champ, Princeton, Harvard or Deluxe? What would it take to “mod” and existing amp to have an Octal Preamp section? Where does one find Octal Preamp tubs that are not metal or microphonic? I want to learn more about this - All thoughts, opinions And experiences welcome!