I think you need to discount some possibilities before splashing out on any new kit. I'm not familiar with the Marantz unit you have but I know it is small (unlike the speakers!), so perhaps you could take that to a friend's house, or a friendly dealer, and test it on their speakers - that way you can check if it is a problem with your source rather than the speakers.
If the Marantz plays fine on other speakers then you need to look more closely at the speakers perhaps. Are your existing cable terminals good and tight with the cable, is there any possiblity of loose cross over wires inside the speaker that may have happened in transit (although both these things would likely affect only one rather than both of the speakers).
In terms of speaker cable what are you using now? Obviously thin 'bell wire' is a no no, but any reasonably sized (and cheap) copper based speaker cable is fine in terms of carrying the width of signal you need. Also, you don't need to spend a fortune on connectors and plugs, just use ones that fit snugly and have good contact with both the cable and the equipment, or use bare wire of course (being careful that none of it strays onto neighbouring terminals). Are the terminals (and plugs if used) on both the cable and equipment 'clean' - if either have become dulled by oxydisation then that will certainly have an impact on frequency response - trim back cable to 'clean' wire, and give the terminals a once over with something like Deoxit if they look a bit dull/dirty themselves.