Level your bass in Mastering with Multiband Compressor
Mixing and Mastering Tip #010
Sometimes during mastering you need to make more than a small adjustment to the bass. EQ is great, but for a more drastic change, one of the best and least destructive ways is to use a multiband compressor and turn the low band up/down. (Independently from the other bands)
All you need to do this is a multiband compressor of some sort (Izotope ozone 7 pictured) and a DAW of some sort obviously
You may also want to turn on the compressor if you are adding more than a small amount of bass, because you are losing headroom by adding gain to the bass. That compression could help you buy it back.
Make sure the gain reduction is triggering, otherwise it isn't doing anything.