As has already been said, speakers and the acoustics of the room are the biggest factors in how it will sound.
With acoustics, the most common problem is a room that is too "live". When you go into an empty room, you may notice an echo. If you can hear an echo when you clap your hands once in your room, your room is too live. The more stuff you put in the room, the less of an echo you tend to have. Having pictures on the walls, bookcases with books, curtains, a big soft sofa, a nice rug, will all be better than a bare room.
Of course, ideally, you would read up on acoustic principles, and put things in place for their acoustic properties. But in practice, just putting a normal amount of things in the room will usually be good enough. A room that looks bare probably has acoustic problems.
Once you have that taken care of, then if you are still not happy, replace the speakers. But you must make the room tolerable acoustically (if it isn't already), because no speaker is going to sound good in a room with terrible acoustics.
As for speakers versus other things, I used to use a receiver that retailed for about $600 with speakers that retailed for well over $6000. It sounded great. Because I wanted more features, I replaced the receiver with one that retails for about $1600. Guess what? It sounds the same, unless I engage a new feature that I did not formerly have. Put your money in speakers.
