How to damper booming bass...

Mmmm, or is OP saying it is still booming? Even after he turns the loudness button off? Any case, sounds like an empty room BOOM,
ay? MC gave some good advise, away from the walls..
Room treatment..as much as you can stand, afford, or put up with.
Do you have tone control? Not running subs too, are you?

Regards

I haven't tried it but i recall reading a principal from (I believe) Magico recommending dacron polyester in the bass ports. Who would be so bold as to say he doesn't know what he's talking about? 

First google "Room Mode Calculator"  Plug in your rooms dimensions.  See where the problem frequencies are and where they are prone to be most audible.

Check that with real world listening tests using a cheap measurement microphone and free Room EQ Wizard software.  If it's confirmed there are two options: first and cheapest is to buy some kind of DSP room correction EQ hardware/ software device that will compensate for anomalies of the speaker/ room system at the listening position by altering what the speakers reproduce.  The second way is to treat the room with targeted absorption and diffusion to break up, disperse, and dissipate the problem frequencies where they are amplified/ cancelled out in the room caused by boundary interference.  If the second method is chosen and implemented properly it will increase the listening quality of the whole room.  An EQ solution can only fix the spot where it is measured.  Moving the measurement spot around the room *may* be able to correct some problems in other places, but unless those problems are in a different frequency band you'll be mucking up the correction at the initial location.

There is also a possibility that adding subwoofers (plural) can add energy where it is low and provide phase cancellation where it is high.  I've only ever used multiple subs to add more bass to a room but I have read about strategic cancellation as well.  While the idea of adding bass to reduce bass seems counterintuitive, it can be done. I would be inclined to hire a profession in any case.

1. PLACEMENT

a few feet away from rear and side walls. Toed In, AND Sloped Back: to avoid initial and reflected sound waves parallel to room surfaces.

sloped back:
a. aim tweeters to seated ear height of listening position
b. woofers angled a bit above floor surface
c. slanted back is similar to speakers specifically designed with ’time- aligned’ front panels. (highs travel faster than lows).

2. LOUDNESS you mentioned ’LOUDNESS OFF’.

Are you talking about a specific loudness feature/filter/switch?

’LOUDNESS’ controls introduce bass boost only intended for low volume listening. It should not be engaged at normal listening volumes. OFF is normal. It is implemented in various methods, not always intuitive leading to inadvertent misapplication.

What is your equipment’s model #?

When set OFF, is there still too much bass?

3. PORT.

I would definitely listen to something very familiar with loudness on and then off. After that, I would temporarily plug the ports, listen again carefully.

I am not a fan of ports, especially if you are experiencing too much, boomy, muddy bass.

4. BASS/TREBLE Controls

Idealists prefer pure signal path, minimum wires, filters, controls ...
Your case is not ideal, and, any time a port is plugged up, the bass control might be the last factor to achieve best mix with midrange driver/crossover which was calculated with the port open.

All the comments about speaker placement are well considered. But it’s worth noting that loudness controls or buttons are rarely well calibrated to do what they are intended to do. The best of them are either rotary controls as on many Yamaha models or are linked to an AV receiver’s setup mic processor as in Dolby Volume. If you have a preamp with a loudness button and a power amp with input level controls, you can reduce the exaggerated effect of the loudness by lowering the amp’s input level. But lacking any of these you have no better option than to turn it off!