I was speaking subjectively. We have several crappy sets of speakers at the studio for testing mixes on a wide range of playback systems. A great engineer can get great results practically no matter what gear he's given.
Basically it's experience. You learn how your room "sounds". You take your mixes out and if they constantly lack bass, then you room has too much bass and you're compensating for it. The opposite is also true, too much bass outside your room and you've got too little in the room and you're cranking it up. If you've got bare drywall or concrete wall, reflections in the room might be colouring the sound too.
As you practice mixing, you have to learn to compensate in order to find out what's going to work on the most systems. It's like a bell curve, at the low end there are always going to be some systems that just sound like crap. at the high en there will always be systems that are so good they'll point out every flaw. The goal is to shoot for the majority of the systems in the middle.
In my mixing experience, it was learning to balance the frequencies and training my ear to hear too much or too little of each band...high mid low etc. I then trained my each to hear certain problem frequencies, like 250htz, and 4-8khz, there is always a build up at these frequencies that can negatively colour you mix. The next thing I try to pay attention to is the balance of instruments in a certain band. Example, are the kick and bass guitar balanced, 'cause if they are, you can eq up or down the whole range and it will still work. But if one instrument is overpowering another, you'll forever be masking the quieter one.
You can train yourself to do this on just about any speakers, the ones I've used in my home studio for the last 15 years only cost me $250 for the pair(Canadian
)And I just know them so well, I've never bothered to fork out for newer fancier powered monitors. I'm spoiled by mixing in a mastering studio and having the best gear available, but that just helps me, I don't have to depend on that.