In answer to your original question, many people, myself included, believe that there are significant sonic differences between tubes and that you can improve or tune the sound of your gear by trying tubes other than the stock tubes that come from the factory. Some people find New Old Stock (NOS) tubes to be too expensive and prefer to buy current production tubes. There are plenty of both available.
"Extreme tube sensitivity" means that the differences between tubes can be easily heard through that particular preamp.
If you don’t want to spend a good amount of time learning about the characteristics of different tubes, it’s best to buy from a knowledgeable, reputable tube dealer who can explain what type of changes to the sound you can expect from different tubes. It would be best to have a handle on the sound of the tubes currently in the preamp and an idea of what kind of changes you would like to hear before you contact a tube dealer.
If you have any other questions about tubes, just ask.