I believe the best and most reliable are Mil Soec Mullards,
Rare and expensive, You will need to find a hoarder.
Mine were in the box wrapped in paper. They are all tested in an active unit before being wrapped.
You need to decide which direction you want to go with the sound. After using the stock tubes, what kind of change do you want, more clarity, airy highs, more bass? There have been good tube recommendations, but they cover the gamut of sonic signatures as well as price points.