You might re-do any mechanical connections such as spade lugs. I don't know what sort of terminals those may have but you'll want to make sure they're solid. As well, if they have attenuators for the mid range and tweeters, these are like rheostats and could use some sort of contact cleaner that removes oxidation on such things. I don't know Marantz speakers or even if Marantz actually made them. From the date noted in another reply, these may have been produced during a time when Marantz had gotten away from it's attention to quality as being its first objective.
Marantz was known for tube type amplifiers and was rated quite highly in the audio world. McIntosh was another in the small crowd of elite products. Marantz was one US brand that faired quite well when transistors began to replace tubes. The Japanese were well ahead of the US manufacturers in this endeavor as they spent a lot of research time perfecting transistors that could be used in audio applications. US manufacturers were cutting corners to compete but Marantz stayed on track by not giving up on quality. What I remember was lifting a Marantz solid state amp and comparing its weight to that of...let's say...a Pioneer. The Marantz of stated equivalent power was much heftier. They used huge transformers in those things for both the power supply and output stages. A classic Marantz 2270 weighed nearly 40 pounds. I remember disputes about power and the Japanese versions stating their's as "peak" while Marantz and the better manufacturers used RMS. A 70 watt per channel Marantz would blow a Japanese 100 watt system apart in a side by side test. Unfortunately, Marantz started to give way to marketing pressure to manufacture such things as turntables and other input devices but I was unaware of their entry into the speaker market. But, as other manufacturers were doing well by putting together entire systems, I'm not surprised that Marantz joined in that game rather than stick with what they did best. I recall that Marantz quality took a hit and just about disappeared. In recent years, it seems to have rediscovered where it came from and has regained some of its old reputation.
Good luck with your speaker project. I know I've seen DIY speaker repair kits on line if you want to research that. You'll need to remove yours from the cabinet and find the model #s. The worst thing that would happen to speakers is a blown coil. Either the windings would come loose due to heat melting the adhesive or they'd actually burn out due to a failed output transistor. If a coil is bad, you find a suitable replacement speaker for both channels.
Sorry to be long winded but I miss my old Marantz 2200 series receiver. It would have cost me more to fix it than to buy a new rig when it finally became unreliable.