Subwoofer boom is too much for me...

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

The best solution is probably, use 2 subs not one.

What kind of sub do you have?   Where do you have it plugged in?