You'll almost never get the "best" tube with a new piece of gear. In many cases the best sounding tube for a particular piece of gear is a tube that has not been manufactured for 50 to 70 years. In many cases it is a major search (months to a year) to find enough tubes in excellent condition to fill just one or two amps or preamps; and the tubes could sell for anywhere from $100 to $500 each because they are rare and not coming back.
Each unit someone makes and sells needs to sound the same, or who would trust to buy it. So a manufaturer needs to find a large, inexpensive, reliable and readily available supply of tubes in order to meet demand for its products.
If some hobbiest wants to take the time to search the world for a hand-full of tubes to change the sound of the gear, then more power to them.
Audio makers don't recommend other tubes for legal reasons. If you play around with your gear and the tubes and screw things up, or blow up the unit, burn down your house, they could be held legally responsible for recommending you try it. That is why every owners manual says that there are no user serviceable parts inside; not to open the top; and return it to an authorized technician for repair.