It does seem like 93-95db is the turning point and they seem to sound better, even when they are not playing that loud. I think dynamic compression is part of it. Inefficent systems with big amps can sound very good but they always sound like they a working for it. A HE system can sound effortless. Another reason why both pro and vintage systems can sound so good is they generally don't make the amplifier juump through so manny hoops. Impedance doesm't drop real low and some have low zmax. I am currently using 8-ohm JBL Lancer 77's and getting quite good sound through a Yamaha M-4 120 watt amp.
I haven't forgotten PiSpeakers and now have access to some very inexpensive JBL drivers. I'll keep you posted!
Dave