There are certain specs for specialized tasks such as film sound mixing but for music it's never really been formalized into specs. The film sound specs include room specs as well as electronic specs and they are intended to facilitate consistent play back in theaters that meet the same spec. That's why some studios run things like EQ on the monitors... it's actually part of the spec.
File that under too much info. :-)
Some say that "monitors" are specialized and flat while "stereo speakers" are meant to entertain and emphasize certain things to enhance particular styles of music.
My thought is that all
good speakers are reasonably flat, while most cheap speakers are not. A lot of times this can be associated with the cheap amps that are stuffed inside them.
Back when we all used separate speakers and amps one could see how a good amp made a cheap speaker sound better. We could also see how a great speaker didn't make a bad amp sound any better... it just sounds more accurately bad.
Now, if you get a powered speaker you get what you get and the cheap stuff will never sound better than it does.
With regards to comparing the car stereo; perhaps you have a nice car stereo? My Honda Van OEM package sounds great. My Wife's Honda Element OEM system sound ugly. Go figure.
With regards to headphones... well, they take the room out of the equation. Batsbrew already mentioned that one should seriously consider the listening environment when considering the usefulness of speakers.
I dislike using headphones and I like to learn to listen to speakers in a room and make the best of it. I have to use headphones for work... I just dislike adding any more time to that experience.
My approach is to keep the speakers off the wall and use all the tried and true placement techniques that stereo buffs have enjoyed for a few decades.
If you take the fanciest speakers and put them up against the wall on a shelf you are wasting their potential. Indeed that is why you can find so many people who claim to have tried the best but also claim that a pair of cheapies are just as good.
If you can't get the speakers off the wall... you are fighting physics... and you'll end up learning about all sorts of compensation techniques.
One thing is for sure, treating the acoustics of the room is always a good thing.
You can use a combination of approaches to make a best circumstance.
I'd be embarrassed to relay how much any one of my speaker rigs cost, so I will not.
I believe that good placement, greater than adequate power, lot's of active listening, and acoustical treatment for the room are the most important factors.
Disclaimer, for those that need to have it spelled out... this is all
just my personal opinion.
I know my answer isn't formatted for you to make easy use of the info.... I'd say beware of anyone who tries to make such a complicated subject seem easy.
In any event, you are certainly on the right track in thinking about all this stuff... I'm confident that if you get enough good info you'll eventually come to understanding that is effective for your circumstance.
If your speakers are sitting next to a wall, try an experiment... get them off the walls and give them a try just to see if it becomes easier.
I hope you find this helpful.
all the very best,
mike