Found the bass culprit in my monitor speakers...

@mijostyn...........................

Bozak did use acoustical stuffing inside their speakers. The walls and back were covered in a half inch thick padding that was stapled to the inside of cabinet.  I should know, I sold and replaced many a woofer and tweeter in Bozak speakers back in the 70’s. 

stereo5, yes you are right. My father had a pair of B302as and as you describe they had I think it was fiberglass insulation probably between 1/2 to 1" thick stapled to the inside wall of the cabinet. They were doing that I think to reduce resonance in the enclosure walls although I can't see how that would work. It would not in any way work in the same way acoustic cotton works in a smaller sealed enclosure. To work the cotton has to just fill the entire cavity. Also in a larger enclosure the pressure (temperature) changes are not as extreme so even if you filled the entire cavity the effect on the systems resonant frequency would be negligible or greatly reduced. The point being that it is not acoustic stuffing in the usual sense, it is dampening of some sort. Maybe there is an old Bozak designer around that could fill us in:)

Mike

Without it, it will make the bass sound boomy

@mijostyn...………………...

Sadly, Rudy Bozak is no longer with us.  I met him in the early 70's.  I drove my company's truck to The RT Bozak Manufacturing Company on Connecticut Ave. in South Norwalk, CT.  I was picking up an order of speakers and I met him on the loading dock!  He showed me around the entire factory (They made all their own drivers) and even treated me to lunch at a Greek diner down the street.  What a nice man.  He had some talented engineers, they are probably all gone now.

@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