From what I remember reading - theres no sonic advantage to aluminimum. Some do ring, and they should be heavily damped. But not all do, such as when the manufacture has made the mechanical resonance lower than the flare rate.

The P.Audio stuff is pretty damn good IMO. I own a pair of P.Audio PA-D45's, and I love em. Even though I havn't compared to other comp drivers - only direct radiators, I can say they really are great. The BM series your looking at is also one thats very good for a "flat and open" sound using those descriptive words, but nonetheless thats what you'll get. I doubt you'll be disappointed.

Sorry I dont know anything about the B&C horn. Perhaps do a search of this forum I think I remember seeing it somewhere before.

High order networks reduce destructive interferance between point sources. The odd ordered ones tend to minmize anomolies caused by the time domain, but theres nothing wrong at all about 4th, 6th order networks etc. Just keep them high order and dont worry about phase shift too much from these as the max phase shift is when driver is already offline.

Hope I've been able to help out some.

Adrian