tl;dr you can run your 6Ohm speakers fine with the receiver with out any concerns. You won't damage the speaker or the amp. If you drive the amp too hard for too long, it will simply shut down to protect itself and cool off.
Loudspeakers impedance rating are nominal, i.e. they pick a single number for the impedance that is supposedly representative of the overall impedance behavior of the loudspeaker.
In reality, loudspeaker impedance is not a single number, and is indeed different at every frequency. The lowest impedance you will see out of a typical loudspeaker will typically be a few tenths of a Ohm above the DC resistance of the voice coil of the transducer that has the lowest DC resistance, and the impedance can go as high as 100Ohm or more in the low frequencies where the driver's mechanical behavior couples strongly to its electrical behavior.
Here the lower line of the first graph shows the electrical impedance magnitude behavior versus frequency of typical well designed two way home loudspeaker. The minimum impedance is approximately 6 Ohms, and the maximum is over 20Ohms, and the impedance changes across the entire frequency spectrum. So impedance is not a single number, and the nominal impedance rating of the loudspeaker tells you essentially nothing.
Now, in regards to the amplifier channels. Modern receiver amplifiers are voltage sources, and pretty good ones at that. This means that they follow ohms law V=IZ. This means that the amplifier supplies the current I, required by the impedance Z, at the given output voltage, V. Your receiver amplifiers have protection circuitry in them to protect the amplifier from trying to put out too much V or I into a given Z, regardless of what that Z is. If the demands on V or I are too great, the amplifier will simply go into a self protection mode.
Most of the concerns about impedances of speakers hooked up to amps comes from the world of tube amplifiers, which are constant current (not constant voltage) sources. There, matching the impedance of the amplifier to the loudspeaker impedance is more important due to the load it places on the amplifier's output transformers. Also, vintage guitar/bass/hifi amps have no built in protection circuitry and can be damaged if overdriven into the wrong (i.e. too low) impedance at high levels.
Buy your speakers, don't worry, and enjoy the music.