Particle board is likely Since AN prefers it to MDF. I believe they use the AN K drivers -- so VIFA drivers and a FOSTER not a fostex tweeter (though the K may also be a VIFA). AN isn't the most spectacular when it comes to appearances so that is no surprise here...so far I've yet to read complaints about these speakers sonics wise. Bass isn't overly deep to about 35hz - that board with the glue is in the center of the box which goes down to another opening at the bottom ( I have seent he AZ three in person just not listened to it). It's similar to the PMC cabinet that has a center board. It seems to be with the reference to panel speakers that like panels it is best avoid the driving rock music as the large port has a fast roll-off.

This guy seems to note the design a little http://www.audionote.co.uk/imagehifizero.htm

And this review -- Well I just find it funny http://www.audionote.co.uk/reviews/t3systemreview.htm