bought a Heresy II off uMay; called Klipsch production date May 4, 1996; couldn't say on efficiency, but 1990 cut sheet says 96dB, last SPEC says 97dB...I also inquired via e-mail, let's see what they say...
it's marked 8 Ohm but no copyright date on the xover...
earlier 8 Ohm HIIs were marked 8 Ohm on xover, but had copyright 1986...