Chances are that the amp will be okay. A decent 8 ohm amp would be able to drive 6 ohm speakers without getting too mad at you. My amp is rated at 200 Watts per channel into 8 ohms, and 350 Watts per channel into 4 ohms. My speakers are nominal 4 ohms but at some frequencies actually have an impedance of about 2.3 ohms. Great amplifiers are able to double their power as you cut the impedance in half. Now, at worst case, an amplifier could be designed for 8 ohms and really get overheated and mad at you if you run anything with a lower impedance. An amplifier like that would fizzle out and die on you over some unknown period of time. In general, there is no direct answer to your question since it depends on the amplifier, the speakers, and how much demand (i.e., volume and listening duration) you put on the amplifier.
