Metal cabinet speakers

@mijostyn, A D'Appoltio Array uses a 3rd order cross-over not a 6dB 1st order cross-over.

I had a set of the Celestine SL600 with the honey comb cabinets and they did some things quite well. but huge power pigs.  had an amazing midrange not so great in the extremes as modern speakers but very musical and engaging if I remember correctly.  but wow needed a lot of power to wake them up 

I have Focal CMS 40 monitors on my desk. Aluminum and very heavy. They don’t resonate and add sound/ambiance like an MDF or plywood box does.

Having said that, a resonate box can be very enjoyable. I love Harbeth and Rega mini monitors. 

Unsound, I am only referring to the positioning of the drivers. Done this way the drivers are acting acoustically as one driver through the entire pass band, an almost perfect point source. Whether or not you use a 1st, 2nd or 3rd order cross and where you put it depends on the physical size of the tweeter as you want to keep the mid/woofers as close together as possible, the upper frequency limit of the mid/woofers and the lower frequency limit and power handling of the tweeter. If you choose the right drivers you can get away with a 1st order cross and still maintain the point source characteristics of the array. It does limit your choice of drivers generally to more expensive ones but it makes life much easier and guarantees a phase coherent speaker. The dipole plate design eliminates enclosure resonance and improves integration with the room acoustically. Placed at shoulder level these little speakers sound really big. With your eyes closed you would swear you were listening to big floor standers. 
If you have access to a CNC mill you could even make the plate out of aluminum.

@unsound Not a metal cabinet, but Coincident has used d'appolito arrays and first-order crossovers in some of their speakers...

http://www.coincidentspeaker.com/eclipse.htm

Speaker features:  "1.The D'Appolito alignment is optimally applied to midrange/woofer drivers no larger than 5.25" in diameter. Any larger mitigates against the optimum spacing between the midrange/woofers and the tweeter, (as measured from the driver centers).
2.First order crossovers are essential to maintain perfect phase relationships and time constants."