I am not familiar with either of the Marantz units but I do see cheap, crummy speaker binding posts on equipment all the time. I suggest if you are handy with electronic repairs, open it up and see if the connection problem is internal. If you are not handy, take it to a good repair shop. It sounds as if the problem is mechanical, not electronic, and should be a simple fix.
Upgrades - the thing is, your ears learn and improve just like muscles when training. Given time, you will hear things and notice things that you would not have noticed before. Only you can decide if the extra kilobuck is worth the sound quality difference but one thing is for sure, don't upgrade if the speaker terminals are the same crummy ones on the PM7001!