Your choice of possible speakers is right on. Other options listed here are great as well.
I approach audio from a "bang for the buck" perspective as well as "risk factor(s)." It appears that the "little" Marantz is going to be part of your life for a while longer. The amp would definitely benefit from power delivery upgrades, starting at the wall socket and working its way to the power cord on the amp. You don’t need to go crazy with this. Things in the "$100 range" can make a notable improvement. When added together can result in (very) significant sonic improvements. Along this line, I would take a good look at your power conditioner/surge protector. A good test is to plug the components directly into the wall. If they sound better, then I’d be looking for something different in power distribution. I’m expecting some push back from those who are not fans of the items mentioned here. This is where the "low risk" element comes into play. If you can borrow these items for home audition (not so easy on wall outlet) you can listen at your leisure and make informed decisions. If they don’t put a smile on your face, then you can return them. (There may be more music in the Marantz than you realize?)
There’s more good news here. These items mentioned are "transportable." When you get your "forever amplifier" they will help the sonics of the newer/better amp as well.
Good luck.