"Dynamic range" and "low frequencies" aren't directly related, but low frequencies at low volumes are harder to distinguish for human ear.
Speaking of dynamic range, if it's very loud all the time, lower it's volume to get more headroom, it'll make your music breathe. Making it louder while preserving breathing should be done by mastering engineers, most labels will get it mastered for you.
As for the lack of low frequencies, it should be done in the mixing stage, your bass should be heard even if you cut everything below 300 Hz, since I can't hear your song the general rule of thumb is to have your bass hearable all the time when it plays during the mix, usually compressor will fix that, and to have the slaps and high harmonics hearable when your song is played through speakers that don't have capacity to play your real bass, raising the highs on the bass track might do that, or you'll need to add something that plays bass notes in other octave but doesn't actually play the bass frequencies.
It's also important to take breaks when you work on a song since your ears get used to the sound you hear, which in fact distorts your perception. During the breaks your ears will reset. I usually listen to Beethoven to clear my ears.
If you have lots of time and energy to do that, I'd recommend to re-set every volume, panning and automation on your song. Then play with the volumes of your tracks until you get it sound at least acceptable for you, bounce it to your ipod along with your old mix, forget about it, and play your song attentively in both variants two weeks later thru headphones, cars, clock radio, you name it, and take notes which parts of the song were best for each setup. When you sit back to work on your mix be careful to have these parts of your song to be the way you liked when you took notes.