Different Tubes and general purpose?

Tube gear manufacturers tend to supply their products with decent sounding, but lower cost tubes that they can procure a large enough supply to fulfill new orders and to have backup tubes for customers wanting them to supply replacement tubes.  Availability and price are key considerations.  That is sensible because any more exotic and pricey tube MAY, but not necessarily will, improve the sound--improvement is a matter of taste and system matching.  That leaves it up to the buyer to decide whether a particular alternative is an improvement.   Ultimately, it comes down to the owner having to either purchase or borrow candidate tubes to listen to the results.

Because trial and error is really the only approach, I would recommend starting with the cheaper small tube types.  If you are new to tubes, go slow and enjoy your amp for many months before attempting to change the sound through tube swapping .  It takes time to become familiar with the sound so that any change makes an obvious difference.  Also, expect to make mistakes--where a change initially sounds good, but in time turns out to be the wrong move.

Most people who have been around tube gear a while can pretty readily hear differences between tube brands and models.  Like any form of connoisseurship, experience matters.  If you are new to wine, it is hard to distinguish, much less reasonably evaluate which is better, without experience and experimentation.

Good luck.