Welcome to AH Jayem, and congrats on getting new speakers
. My comments below echo the advice given by others above. Because you asked numerous and detailed questions, I thought I'd answer each of them.
I just recently purchased Sonus Faber Sonetto Vs and Center 1 for the fronts of my 5.1 system, upgrading from some Mordaunt Short bookshelf speakers I've owned for over 15 years. While I'm waiting for the Sonettos to be delivered, I'm looking at my Marantz sr5013 and thinking I'm almost certainly going to need an upgrade to drive these 4 ohm speakers. The guy who sold me the speakers recommended I give them a try on the Marantz before I jump to buying something new, which I respect since he could have tried to sell me on something else.
The advice given by the guy who sold the speakers is good.
Your Marantz receiver is rated at 100 W at 8 Ohms / 20 Hz to 20 kHz / 0.08% THD. It is said to be stable at 4 Ohms, but no power rating is given at 4 Ohms. You can expect it to be somewhat more than 100 W.
Sonus Faber recommends amplifiers capable of 50W – 300W to drive these speakers. Your AVR can do that. SF also says the speakers should be driven without clipping. That is true for any speaker/amp combination. SF claims these speakers have a sensitivity of 90 dB, probably a slight exaggeration. As a guess, I'd say these speakers are not difficult to drive, and your receiver can drive it. Without seeing a graph of impedance vs. frequency, nothing else can be said about how easy or difficult your new speakers will be to drive. I looked and couldn't find such a graph.
Still, I keep pondering- am I wasting these speakers by having them paired with the Marantz vs. getting separates or maybe just a better AVR? I've read lots of posts on this forum looking for a clear answer regarding what to expect. I've read posts here that say that increasing wattage won't make much difference if you don't go loud and doubling wattage only increases volume by a few decibels. I tend to listen to things at a medium volume and am more interested in sound quality than volume itself. I'm positioned about 10-11 feet away.
That info is generally correct. But wait and see if your receiver can drive your new speakers in your room.
Options include sticking with the Marantz, adding an amp and using the Marantz as a preamp (which I have read may not be ideal), or getting all new separates.
If you do get an external amplifier, you can easily use your Marantz receiver as a preamp. Ignore any advice that it is not ideal.
When getting separates, is there much difference in sound quality with more powerful amps?
In short, no.
What is the advantage of higher end amps over lower end ones: noise, reliability, or sound quality? Do I need more power if I don't plan on destroying my eardrums?
There no advantage of higher end (price) amps over lower price amps. Well-designed solid state amps operating well below clipping, that are not misbehaving, and which have high damping factor across the band, will tend to sound the same. Note that some very expensive solid state amps do not meet this criteria – sometimes as a result of misguided design decisions intended to "improve" the sound.
Or is the benefit in buying separates that I will have a better preamp (better DACs/other components)?
No. There is nothing wrong with the preamp and DAC sections of your Marantz receiver.