dynaco st 70 what tubes to replace first

Ive just bought a rebuilt st 70 and am very happy with imaging and sound quality Ive checked the tubes that are with it and they are run of the mill. WANT TO UPGRADE !!

Suggestions please on what order to replace first and if you can; recomendations, Im looking for a creamy thick sound.

Im an old guy and this is my first tube amp and limited knowledge but a fast learner and very excited to go back to non-ordinary reality via sound.
Larry