Audiotent Tip 367. Adding Weight To Your Instruments.
Here are three great techniques to add weight to your instruments.
1) Use an equaliser to boost the low-end of the frequency spectrum. Anything below 200 Hz can thicken your sound and make the instruments appear heavier.
2) Compress the low frequencies using a multi-band compressor. This is a great way to thicken the sound which has a dynamic low end. Compression will keep those lower frequencies under control and you will also save some headroom.
3) Layer with another or the same instrument that is one or two octaves below. Sometimes the lack of weight has less to do with the tone of the instrument, but more to do with the notation that it plays.
For example, you can't expect the synth bass to be deep and heavy if you're playing it high in the key register. So, if you have the option, pitch the sound down to add the extra weight. If not, add another layer which contains the much needed low-end energy.