Metal cabinet speakers

Aluminum is an ideal material for loudspeaker enclosure. Very stiff, low energy storage, and therefore easily dampened (most of these cabinets are well damped). If you ever knocked on a Magico or YG, you know that they are as inert as cement. 

@anotherbob I think the mythos is MDF... but does have a granite base.

@sciencecop Thats what I suspected. Maybe thats why Magico is so loved?

@mtdining Have you heard the MK2 series or M series? The M2s really impressed me. They sounded super detailed but still musical.

kenjit, it is easy to build passive cross overs. First class parts are ready available and the math is not that complicated but unless you stay first order phase coherence and efficiency matching drivers with various impedance curves without adequate test equipment is virtually impossible. The easy way around this is to use active cross overs and bi or tri amp. My favorite way of making a very high quality system for friends with a limited budget is to make a D’Appolito array using two 6” drivers up an down from a dome tweeter on a plate sandwich of MDF and solid surface material. I’ll use a simple 6dB/oct cross over and tweak it measuring the frequency response until I get it reasonably flat. Then I’ll cross over to subwoofers at around 125 Hz. You can mount the plates on stand but my favorite stunt is to hang them on chains.

It looked like the ProAc K8 has a metal structure. I found out when I had to remove a bass unit  to replace a broken speaker wire terminal. 

As Erik_Squires noted, Celestine SL600 and SL700 had cabinets made from Aerolam. Two sheets of aluminum with aluminum honeycomb in between. Adapted from aerospace industry. Inert and light in weight and thin allowing a relatively large internal volume. But very expensive and, therefore, used only in small speakers.