The ear perceives distance because a sound that is felt far away has less treble and when it is close it is treble richer. Consequently, a sound with more bass is perceived as close if you can define it, but if it just rumbles it feels further away and if it should feel very far away, it loses in the bass register as well.
Just as you should work with placing sound in width, you should also work with placing sound in depth as well. The goal is to create a three-dimensional sound image with two speakers.
To take an example: you have two guitars and one you want a feeling of closeness, and then you try to find closeness by developing and boosting frequencies between 2.5 kHz – 9 Khz, but be careful that the guitar does not becomes too thin and hard. You may want the other guitar a little further away and then choose to remove a little in the same frequency range to move it further back in the sound image. Test on different instruments and see how far away or how close you experience them when you try the different variants.
Here you will find the first part in how to create depth in your mix with reverb and delay