Subwoofer boom is too much for me...

Also think placement then crossover setting/gain key. Perpahs four is too many?

Much has to do with the quality of your sub , I went from the $500 Svs pro 

to the 1000sb $1000, to the $svs 4000 sb $1800

and the 1000 had much more control and articulation ,

then to the 4000 SB-Sealed box. It took it to another level altogether the bass much faster,more articulate and controlled. ,No boom whatso ever .

you didnot mention what subwoofer you were having issues with.

Try a lower level for the sub ...

I've done this on three subs with great results. Had the same issue as you describe. Now I have a subtle more controlled bass response. I just used sturdy paper tapered coffee cups pushed in snug. Washcloths didn't seal enough for my liking.  Some people have cautioned against this as they worry the sub amp will overheat without the designed airflow in and out of the port. Since I am trying to tame the boom I don't have the power on it too high to begin with and haven't noticed any heat build up even after all day of running this way. Good luck I think you'll like what you hear. If you have the budget a sealed sub is what you are after. This "conversion" creates a sealed sub but with a typically larger enclosure than a sealed sub would be in.  

You can add an equalizer and just turn down all the sliders especially on the higher end and the low end you can lower the sliders to reduce the bass. It works I use an eq and achieve the same results