Hello, this is Yoav Geva, of YG Acoustics.
A lot of words have been spilled on the subject of cabinet materials, and since I have done quite a bit of research on this matter, I feel that I have something to contribute.
The main thing that I can contribute here is that measurements of arbitrarily-sized individual sheets of various materials really dont tell you enough. If you want to know how a material behaves in a speaker enclosure, the best way to do so is to build the best enclosure that you can with that material, and measure the resulting end-product.
When doing so, one should use the fastening methods most appropriate for each specific material, and varying thicknesses which are optimized for every different panel within the enclosure. This is a process which takes several years, and is essentially the equivalent of developing a different full-blown product out of each of the materials one is considering.
I actually performed this process, and have ended up with a combination of aircraft-grade and ballistic-grade aluminum alloys (all from the 6061T651 spec, which by the way is superior to 6061T6), and special pressurized-assembly techniques to keep the structure under tension (by the way, aircraft is also made using pressurized assembly, so I have to admit it is not my invention in any way).
On a more personal note, please realize that I (and other designers who use aluminum) are not in love with the material. It is expensive, takes time to machine and is hard to perform grinding and finishing on (I would have LOVED to avoid the cost of my Portatec CNC machining center and Kuhlmeyer CNC grinder). I wouldnt have selected this material unless it had been proven over and over as the most suitable material for a highly-rigid, non-resonant enclosure.
Since I dont expect you to just take my word for it, I would like to back this post with actual measured data. May I please direct your attention to my ad titled Reason #2, which appeared in Stereophile of July, August and December 2008. It showed a vibration analysis of my flagship speaker (Anat Reference II) versus the flagship model of a leading competitor who uses resin for the bass and a laminate for the midrange. Out of respect for the competitor, I would prefer not to mention their name. In this comparison, it was clear that the aluminum enclosure was superior to both materials used by the competitor, by a considerable margin.
And for those of you who are curious about seeing more vibration analyses, I can reveal that our future ad titled Reason #6, which will appear in Stereophile of August 2009 and The Abso!ute Sound of September 2009, we include a comparative measurement analysis with 2 additional competitors: the leading European competitor (wooden bass enclosure), and a competitor with significant press attention that uses machined aluminum baffles and a wooden enclosure. I believe that once those measurements are published, this debate will be given objective answers.