Should I use Impedance 4 or 8?

Maybe someone out in Audiogon land can definitively answer a basic impedance question for me.I own Klipsch Lascalas paired to a Raven Audio Blackhawk. Obviously they are rated as 8 ohm and the Raven provides both 4 and 8 ohm taps. The impedance tests on Lascalas show the impedance going very low at certain frequencies. (Like 3 at some frequencies) I've tried the 4 ohm taps which does squeeze a tad of hum out of the tunes but it seemed flatter and duller.Any advice? Will I damage my amp? Why the downgrade in dynamics at 4 ohms?