Speaker ohm's?????

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.

It's always a good idea to match your amp to your speakers. As far as I know the damage will only occur to the amp if you are requiring it to drive a lower impedance load than it is designed for. Nominal means average, which means the speakers can drop lower usually a couple more ohms.

-aj

Its unlikely that "6 ohm nominal speakers" would hurt any amp. Also, you are calling it an "8 ohm amp" probably because the specs rate its power at 8 ohms and don't bother to list its rating for lower impedances, but really, all amps will drive any impedance down to maybe 4 ohms, but possibly at less power. Don't worry about it.