Yes, Elevick, if I were to use ribbons, a crossover would be necessary; in my experience they don't hold up to delivery of low frequencies except at Low volumes. As a matter of coincidence, the tweeters installed in these enclosures are EMIT ribbons. The use of piezoelectrics, though, are a somewhat different story assuming that my experience with them is representative of their character across the board. Back in those days I was a teenager and used to listen at fairly high volumes and never destroyed one.
Your point, though, concerning the volume difference reminds me that when I used a piezo, it seemed a bit overpowering. If I were to attenuate anything, then, it would be the tweeters and maybe adjustable resistance to them would be in order.
The impedance issue that you mentioned remains a concern but to answer your question, the amp that I'd be using, although of mediocre quality, is very high current capable.
Tim, your answer is exactly what I wanted to hear. At this point I have neither driver that I would use and the ones currently installed will be removed along with the adjustable X-over. So I'll be starting from the ground except for the enclosures which are the old Phase Tech PC 60's. Your description of the mods is very interesting and reassuring especially in light of my original experience where I found the them to be effective but a little "scratchy" sounding. The physical attenuation you describe, if I understand it, would not interfere with those frequencies in the audible range as long as the amount of added material was controlled to avoid too much attenuation. I assume that this determination was based on some amount of trial and error. BTW, I was surprised to see that the roll off was as low as 1800 Hz. Also, when you put them back together, did you replace the resistor and, if not, why was it removed?