I recently purchased a SSA Evil 18in sub, and a 3k rms amp. my first ever real set up so im pretty new to car audio. but i have a question and in need of advice. when i turn up the volume it almost sounds like the subwoofer is bottoming out or making a clicking noise. is this normal or do i need to lower the power on it. or  is there more to it than just that?

Could be a ton of different things. That being said if a sub is making any noise other than bass something is up. Leaks in box, bad port area etc.  Someone with more experience can chime in but i wouldnt push it with that noise.

Try to turn your gain all the way down. What's the max volume of your head unit? Go to a little less than 3/4 of max and slowly turn your gain, or input level up. Idk what your electrical and all that is like so try to stay less than half way up on the gain. And turn the bass boost to zero. It's uselesss and false leading you into thinking your sub is getting louder. I ordered a pair of 12s and I'm excited to see what they are all about .hope it sounds better 

How was the gain initially set?  Are there any leaks around the gasket?  Where is your SSF set?  What are you tuned to? How big is the box?