How to damper booming bass...

The first step with ANY sound issue is finding the best placement of the speaker.  While moving speakers away from the corners of the room will reduce boom, a more precise way to find the best placement is to use one of the methodologies that involve systematic placement experiments.  Look up the "Sumiko Method" of speaker placement.  This involves moving the speakers in very tiny increments to find the location where bass is in proper balance.  You will be surprised by how even a movement of less than an inch can make a dramatic change, and because room modes can be all over the place, it is not necessarily the case that the further from the corner or walls one goes, the less the bass boom.  Likewise, experiment with placement of your listening chair.  

As others have mentioned, experimenting with a change in loading of the speaker by stuffing the port is also a worthwhile, and cheap, tweak.  After that, there are more costly approaches such as DSP room correction, subwoofers, room treatment with bass traps, and so on.

Assuming your speakers and chairs are up against walls Millercarbon's suggestions will work. Stuffing the ports will change the bass depending on the tuning. It will cut the lowest frequencies off almost entirely. But, stuffing the ports with towels is a harmless easily reversible thing to do so why not try it! That's the way you learn. 
If you do not like the bass than these are probably the wrong speakers for you. Unless you can afford digital equalization you are better off buying new speakers than spending money on an analog equalizer that will mess up everything else. 

Headphones?

Location of the speakers.

EQ.

Bass traps.

What speakers are they?  For many, only some mitigation is possible.