Found the bass culprit in my monitor speakers...

@audiodav

A lot of good comments, I am not familiar with your speakers, but I assume that you have sealed box woofers, the comments are reflecting such. I would recommend that you do a search and read about the spec called QTC. This spec reflects the smoothness of your bass using a woofer in a sealed box. A flat response has is a qtc of .707. As your box gets bigger Q goes down, then as the box gets smaller, Q rises. So in a larger boxQ will drop, below that number, say .6, your woofer will roll off faster and eventually takes a dip, your speakers lose deep bass and develop a dip. As your box size goes down, Q rises, a Q of 1.0 will have a significant hump in bass response normally causing boomy bass.
I prefer a QTC of .7 to .8, but many designers like a lower Q to design for room settings, I’ve seen speakers with a QTC as low as .5. If your speaker had a Low QTC, It is possible that when removing the stuffing that you raised Q to a more flat consistent output.... I really don’t know this about your particular speakers, but it is certainly a possibility.
You can slowly add fill to your boxes, as your boxes become stuffed, as others have stated, it will act as though you are getting a larger box, once you get totally stuffed, the stuffing has the opposite effect, once are travel starts being restricted, stuffing starts reducing air volume in the speaker.
This is the basics of how and why, I hope this helps,
Tim

Another, less technical, way to think of it- not that any of the above is incorrect, its not-

The sound coming off a driver doesn’t just come out the front and into the room. It goes off the back and into the speaker cabinet as well. So now just imagine if there was another mini speaker playing music inside the cabinet. For sure some of the sound it makes is gonna come out through the driver and you will hear it.

Well, that is exactly what happens with every speaker cabinet. The drivers send just as much sound back into the cabinet as out to the room. The sound that goes into the cabinet, by the time it bounces around and comes back out the front there’s only one word for it: distortion.

Now sometimes at certain frequencies this distortion is just right to actually reinforce the same frequency coming off the front. When this happens, if it brings up a dip in response then we are happy and say the speaker is flat. But if it reinforces a peak we complain and say the speaker is boomy. So another example of how people can prefer even something as seemingly obviously bad as distortion.

But its not just the low bass that comes out. All the sound at all the frequencies bounces around in there, and the less that’s attenuated inside by stuffing the more that’s gonna come out the front.

Any one particular individual might like this distortion. With his ears. His music. And equipment. In his room. Speaker builders usually try and design for a wider appeal. That means designing for lower distortion, less color, more neutral tone, etc. That’s why they stuff em.

I'm sure. My father's B302a speakers were a glorious part of my childhood. With a Dynaco stereo they would play 95 db or so beautifully.
He had an Ampex real to reel and the prerecorded tapes he had were incredible, a lot of Jazz and classical. I miss the hiss:)
What a dummy I am. I spoke to an old friend of mine who was into Bozaks. The insulation was to kill resonance between 500 and 600 Hz, the wavelength of the inside of these large enclosures! I should have thought of that. 
Millercarbon, I'm afraid it does not quite work that way accept at certain frequencies that keep ringing after the cone stops as in the Bozak example above. Inside the enclosure sound waves are pressure waves that at low frequencies help determine how the cone moves. Sound from the midrange driver which is open in back in the Bozak does not leak out through the woofer cone. The woofer cone is too stiff and heavy for that. Very high frequencies might but in this case the tweeters are mounted on a bracket out in front of the woofer. At low frequencies the whole cone moves influencing it's frequency response. The enclosure vibrating is a problem and is a form of distortion. In the B302a Bozak this created the very warm bass that a lot of people like my father loved. That enclosure was a musical instrument! It was just 3/4" plywood without any bracing. Nobody would make a speaker like that today. But in that Bozak it was euphoric as hell. 
Now I am all for Room Control which is actually speaker control. Most people use it in subwoofers only and downplay it for full range use. I use it to equalize the satellites so that they have absolutely identical frequency response curves right out to 20 kHz. The result is pristine imaging. No two identical drivers are exactly the same and no two drivers occupy the exact same place in space. Two identical speakers in two different locations sound different to various degrees. We locate sound sources by differences in volume and phase (arrival times between ears.)
So, if you want a voice to image dead center the sound of that voice has to arrive at both ears at exactly he same time at exactly the same volume. If the volume of various frequencies contained in the voice is louder in one speaker than the other you essentially dissect the voice and spread it out. The image becomes bloated. Most people try to control this with room treatments. It is much more accurate to use digital speaker control where you are actually measuring the exact frequency response of each individual loudspeaker and correcting them so they are exactly the same. Works great:)