@Captainhemo can reply with the specific material. He told me it was a polycarbonate composite.
It's actually a Poly - Urathane composite, slightly denser than high quality MDF. It is very stable and basically water resistant, great edge integrity and when it is glued, it is actually fused so panels basically become one... no seams to show up down the road.
Unfortuanately, it is not cheap nor is the machining... each of those bases starts from a 24" x 24" x 6" solid block ... then spends approx 24 hrs on a 5 axis machine.
Do a search for " Updated Line Force" , a thread from when we first re-designed these speaker cabinets and searched for a composite that suited what we were after.
We put a ton of attention and detail into these, no edges left unfinished, everything finished with a nice tight 1/8" radius aside from verticle edges on front baffle which are .5".
Anyway, lots of info in that other thread
jay