I picked up a ROKU, I am wanting to cut the Cable TV cord.

cable companies only have fake nooz, fake sports and fake movies so save your $90/mo

This response only pertains to telephone costs if you use a landline......After checking all of my local providers to see what telephone would cost whether with a bundle package or not I looked into the best  option at the lowest cost.
OOMA came out on top above all the others. I suggest going to their website for details. It is an Internet based system that requires a broadband connection.
The base unit is $100 if purchased online and not at local stores. Any phone (wired or wireless) can be used but I got their wireless handset ($50) and the optional wifi adapter ($40). Your current phone number can be ported over to the system for a $40 charge. You can select a temporary number until that happens. Mine took a bit less than 2 weeks.
I only subscribe to the basic package since I don't do International calling but there is unlimited calling within the US.
That may sound like an expensive initial cost. My former bill from the local carrier was $50/mo. Just for phone. With the OOMA my bill averages $4.50/mo. Doesn't take very long to recoup the initial cost.
 

I think I remember somewhere reading that Marantz receivers have some kind of compatibility issue with Roku. Could be wrong but go to AVS Forum to their Roku section and ask the question. I have two Rokus through two separate Yamaha receivers and never had an issue. I own the Roku 3 and Roku Premiere +. The new Roku Ultra seems to have a number of glitches reported on the Forum.

Wanted to answer the question.  The reason you see buffering on some services and not others is simply the quality of the engineers and how many hops you are from a server.  In late 2016 ISPs estimated that Netflix accounted for 37% of all traffic during peak hours.  Netflix is hosted on AWS, which is the same as Prime Video...but Netflix is neurotic about availability.  So much so they developed a tremendously cool piece of code called Chaos Monkey.
http://whatis.techtarget.com/definition/Chaos-Monkey
If you are curious about the topic read up, but the tldr version is that they have a team that does nothing but make sure when things break everything continues on like absolutely nothing happened.

I cut the cord about 3 years ago, it has not always been easy but my favorites are:

phone:  https://www.obitalk.com/obinet/
tv over the air: https://www.silicondust.com/
and for the WAF: we run all Apple TVs (I have a hidden amazon fire)
We pay monthly for Hulu, Netflix, Prime and HBO.

@mattg3,

 Thanks for the response. Like I said in my OP Amazon Prime plays movies flawlessly If I feed the HDMI from the ROKU directly to an HDMI input on the TV.

I'll try doing a search on the AVS forum about incompatibility problems between the ROKU and Marantz HT receiver. It is strange though the HDMI would cause a bandwidth problem loading an Amazon Prime movie.

The Samsung LED TV only offers a Toslink digital output. I'm not a fan of Toslink but I guess I will try it from the Samsung to the Marantz and see how it sounds.  I need to pick up a 1.5M cable.

Which ROKU do you recommend for best picture and overall quality?
The one I am using now is the 3. I assume that's what the big 3 on the top of the box means.

By change are you using Sling TV? If so how is the sound and picture quality? Is the picture quality comparable to Cable TV?

Jim