hz is the unit of frequency, my friend
dB is the expression for the sound pressure level.
In my opinion you want to know how you should level and eq a sub bass so here are my tips:
Equalizing a bass synthesizer is highly individual because every Bassline is different from each other.
Try to find frequencies which stick out that you don't want to hear.
Try to accentuate the lower frequencies and cut out the higher ones, except it doesn't sound good.
The perfect level of your bass mixer track depends on your overall mix and listener preference but you should stay below -9db.
Also, if you use 808s you can use a Saturator plugin to boost the sound of your 808.
Certain plugins can make your sub bass or 808s go crazy, trust me.