The rule varies a lot with different authors and at different times. I'm old enough to have decided there is no use in following the changes. Use your own ears to find what enables you to hear and enjoy the music! My own rule is to determine what is the weakest link in my own system and work on that until something else has become the weak link. The fastest way to learn is to listen to many systems, so it's usually worth finding a local club if possible (or starting your own, as a number of Bottleheads have).
Personally, now that I am using SET systems, the speaker and room is the weak link. I use the singular because they are especially inseparable - you need a speaker that works well with your room, in a location that you can actually use. Speaker reviewers who do not live in your house are of limited use. Not no use, but limited.
Sometimes room modification is more important than a better speaker, and sometimes it's the opposite. Given the low power of SETs, your speaker choices are limited to those with enough efficiency to be loud enough, which means a good proportion of your effort will go to finding the ones you like.
I say all this as someone who makes a modest income from amplifiers, and nothing at all from speakers. Next time anyone promulgates a rule of spending, try to find out what they are selling!
