All good questions. Regarding types, either read your manual or pop the lid of the pre-amp. You'll then be able to tell what tubes are used. Then listen a lot to the sound of the pre-amp and ask yourself - what don't I like or what do I want to improve. Then post that specific question here or in Audio Asylums tube forum. Reference both current production tubes and NOS. Go to Audio Asylum and read Joe's Tube Lore in the FAQ section for a history of tube sound of the different type of NOS tubes - not definitive, but an excellent place to start. Also, become a regular reader of the Tube forum in AA. A lot of good info passes thru that site. Regarding up-grades, at the price level of your Anthem pre you might be as well off buying a more sophisticated SF product such as the SFL2 than put mods in this unit.