OK, I'm still out of town (in Shanghai), but can answer...
Power compression is simply the DCR of the voice coil increasing due to heating. Aluminum actually has a LOWER temperature coefficient than copper, meaning it changes less as you heat it than copper. So it's a bit better in terms of power compression.
Power compression, overall though, isn't a big issue from a distortion standpoint. Essentially it increases the DCR of the system, meaning that the entire efficency curve drops. It's like turning the volume down a notch on the system. It's a linear effect.
As Neil posted, the BL compression is a BIG problem. You can think of this as a compression that INCREASES as you move away from the center. It's independent of power, independent of thermal issues, and happens EVERY CYCLE. Power compression can take time to start happening; BL compression is immediate!
Overall, though, the compression differences between Al and Cu are so small that I would consider them insignificant. We're talking a tenth or two dBs in differences when both are run hard into compression. Not a big difference at all; certainly not one worth considering.
The big thing with Al is just it's mass; if you want the lowest mass, go Al. If you want maximum BL and packing density, go with Cu. Choose between the two based upon the tradeoffs of efficiency and BL that you need.
Dan Wiggins
Adire Audio®
