Keep in mind that the Frequency range of most speakers and the human ears goes from 20Hz to 20000Hz. 40 – 200Hz can be considered as bass, the voice is around 1000Hz (therefore our ears hear best there), high hats are around 5000Hz – 12000Hz, and above 12000Hz you actually have no specific sounds but the upper harmonic contents of any sound used.
To have a round and pleasant sound means to equally use the whole spectrum from about 50Hz – 200000Hz. This doesn’t mean that you need to fill it up completely, but that you have at least a little bit at different points of the spectrum and not everything around 300Hz and 1000Hz, which would sound shitty.
Try to look at the spectrum from 20Hz to 200000Hz like a highway where every frequency is a a seperate lane. As it is on a highway, there can not drive two cars on the same lane at the same point. So what you need to do while mixing (by using the equalizer, the choice of sounds and octaves and the arrangements) is to take care that all cars drive either behind each other or besides each other.
For example: If you have two electric guitars playing on the same octave and playing at the same time very different patterns it will most likely mess up your mix and make it sounds bad. What you can do now is to equalize the guitars so that they don’t disturb each other anymore so much(one sounds more bassy, the other more midy) or you can have one of them play one octave higher or, simply, they just don’t appear at the same time in your song. The good news is, that you have twice that highway. One on the left side and one on the right side. So you could also try to have one guitar on the left and one on the right.
And thats basically what you do when mixing. You have to make the sounds come along with each other. A lot of it is done with the right arrangement and the equalizer. Use the highest quality equalizer you can get and rather lower frequencies that distrub than boost frequencies you like.
EQ- Plugins differ very much in sound. URS and Voxengo sound great. UAD of course too. If you feel like you are not able to do, what you want it might be, because you use not the right EQ for the task.