A good track usually has a nice clear sound down in those bass frequencies which doesn't sound muffled. Many mixes from beginner producers struggle to get a great sounding mix and complain of a muffled sounding mix in the lower frequencies. Usually the cause is that too many instruments are competing for the space within the bass.

A commonly used technique to leave room in this area is to use some low cutting of EQ or a high pass filter on certain problem instruments. For example, imagine we have a bass guitar and a standard electric guitar. When both are playing together, the electric guitar will still be producing some frequencies down in the range where our bass should sit. Applying a high pass filter (cutting out the low frequencies) on the electric guitar can remove these often unnecessary frequencies that the electric guitar is producing and thus allowing the bass guitar to push through much clearer.

By applying this technique to the different sounds that are occupying these low frequencies and shouldn't be, we should start to get much better mixes in the low end, leaving those frequencies free for the things that need it (the bass and the kick). I don't want to give out specific frequencies to cut as you should do this by ear. There is no key frequencies as every mix is different. You want to cut enough to let this technique work but not so much that it ruins the individual sound of each part. If we cut too much bass out we might start to lose the feel of the track or start to get a tinny sounding mix.

Sounds may even have some frequencies in the bass which we cannot hear audibly, but they are there taking up a slight bit of room. With lots of different sounds playing at the same time this can soon add up and it may even be worth cutting even though you can't hear it doing much.

If you haven't tried this before, try it on your next production and test the results for yourself comparing a mix using low cutting compared to one with no low cutting.

Final Tip: If you are doing an EQ cut down make sure that you compress before the EQ cut. If you compress afterwards you may be raising those frequencies up again with the compressor.