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.
Subwoofer boom is too much for me...
Could I tone down the boom on my subwoofer by plugging the port with something like a washcloth? Have you ever tried this and had success?
Thanks for your thoughts.