Good answer from Mooly.
Even so-called 8 ohm speakers will dip below that. The rated impedance, 6 ohms in your case, is a nominal figure. Magazines that show technical stuff that WHF avoid printing reveal as much. If you use a search engine to find 'speaker impedance curve' and look for images, you'll find graphs that look like big dippers. That helps show how nominal impedance ratings are.
Your Marantz will have a rated power output into 8 ohms and 4 ohms. The 4 ohms output should ideally be double, but in modest amps usually yield about 50% more. But your speakers will be no problem at all, and sound quality will be unaffected. If driven absolutely flat-out for a long time, the amp might trip out for safety, that's all.