What next?

Excluding the front speakers(Monitor Audio PL500) what has the most impact on surround sound? The center, rears, or AV receiver. My center is a Monitor Audio Gold C350 and I'm thinking about going to the Monitor Audio PLC350, but won't have an opportunity to hear it. My rears are Monitor Audio Bronze and I could go to Monitor Audio Silver speakers. I have a Marantz SR5011 AV Receiver and could go to a better AV Receiver. What is my best option?
Are you using only the Marantz to drive your speakers?  If so this is a no brainier.  Your speakers at the very least deserve a separate power amp and high quality prepro.  Is this system 100% home theater or is stereo and/or surround music important as well?
I have a 2-channel and surround set-up.The Marantz only drives the center and rear speakers. When I'm listening to 2-channel the Marantz is turned off. My two channel consists of the Jeff Rowland 625 S2 amp, JR Corus preamp, and JR Aeris DAC. I use my HT bypass on the Corus when I'm watching movies, so the 625 S2 is still driving my fronts. 
Since it is the consensus that speakers make the biggest difference in a system, and that the center channel gets 80% of the movies' audio soundtracks, it is easy to conclude that the center channel would have the biggest impact.
I think that having the same amp driving all front 3 speakers if vital.  It's the only way to get seamless sound across the soundstage.  To match your Rowland isn't easy or cheap but will make an amazing difference.

I know that for music, the Rowland will crush the Marantz but, how does theater sound when you use the marantz for all 3 front speakers?  Probably much better for movies?
I don't use the Marantz to drive the front speakers. The Marantz drives the center and rears only. The Rowland drives the fronts rather I'm watching a movie or listening to 2-channel. 
It's too expensive to try to match my Rowland, so I opted for the AV Receiver.