Question about speaker impedance

You're fine, go ahead and rock out.

Just about every known receiver and amp is compatible with speakers with 8 ohm or 4 ohm or anything in between. Those 6 ohm speakers will work fine with the Marantz.

You may be thinking of the scenario in which two speakers are connected "in parallel" to the same amplifier channel, in which case their combined nominal impedance would indeed be 3 ohms. Fortunately, you are just over-thinking things. If your integrated amp has two separate channels for stereo left & right, as I believe it does, then all you need to do is connect one speaker to each channel, and you should be perfectly fine (6 ohms for each channel).

By the way, speakers tend to vary in impedance depending on the sounds they're reproducing anyway, so in basic terms everything doesn't have to be SO precise. As for power ratings, the speakers' ratings are how much power they can take (supposedly), while the amp's rating is how much it can supply (again, supposedly, and usually per channel). In practice, the amp will provide only as much power as you need at any given moment, which for most people is not very much on the average, like maybe 1-10 watts most of the time (with a higher power rating still being important for dynamics and certain really loud sounds). The speakers and amp do not have to be matched in this regard, either--those are merely their independent, respective limits.

Many AVRs are rated for 6 ohms minimum and they'll shut down in a heartbeat with a 4 ohm load, or even a nominal 8 ohm load if the particular speaker has impedance dips that go close to 4 ohms. Since the receiver the OP is considering is 4 ohm rated he should be OK, but you can't always assume so. B&W for one is notorious for having some 8 ohm rated speakers that are difficult loads for 6 ohm rated AVRs.