how to set up good home theater sound (and picture)

Another thing.  Putting linear power supplies on your FIOS One and/or Roku type streamer will significantly improve the audio/video quality of these devices.  However, none of these streaming devices are going to hit the same audio/video quality of a bluray/4K disc.  Even though your streaming services will deliver 4K content, the video data is going to be more compressed.  The audio streams are still going to be old school compressed Dolby Digital or Dolby Digital Plus.

I actually have quite a bit of respect for Marantz processors.  They are the only "consumer level" processors that have a discrete analog output stage.  Excellent power supplies.  It provides a very refined and full sound with lots of impact.  That being said, the Marantz is voiced slightly warm and will have soft rolled of highs, but only in comparison to another processor.

So, if you want something with better high frequency response, the Anthem is the way to go if you want 4K and all the bells and whistles.  A step above this would be the Krell Foundation 4K, but we are getting expensive now.

I would agree on avoiding Emotiva/Onkyo/etc.

I don't have direct experience with Roku so I couldn't comment on Erik's experience, but my general assumption is "HDMI - audio return channel" is a decent feature but not necessarily as good quality as a with a direct source.

If you are using a "FIOS TV One" box, it is likely that you can upgrade that external wall-wart switching power adapter to a linear power supply. Just make sure you figure out the exact size of the DC plug and the voltage used (such as 5V or 12V or something). You can find a china linear power supply on ebay for $100-200 dollars. These are decent enough to give a big improvement over the switching power adapter. Just as a side note, I have seen a significant improvement on audio/video quality for a Tivo by adding a 12V linear power supply for it.

As far as PS4, I don’t know. It uses an internal switching power supply with multiple voltage outputs, so there really isn’t anything you can do. I believe it was engineered mainly as a gaming platform with media streaming support as well. It’s definitely not an "audiophile" device. If you wanted to stream with an audiophile level, I would look into something like a Roku device, then upgrade it to a linear power supply (BIG improvement). Or a Tivo. The Amazon FireTV cubes seem to have a weird power connector, so I don’t know about that one.

Googling, I see that there is a " pendoo Android 10.0 TV Box " using the latest Andriod 10.0 operating system available from Amazon for $55 if you want a more generic "android" streamer (instead of a more prioprietery Roku or Tivo).

The HDMI cable also has significant impact on sound quality. Just because it’s "HDMI 2.2" or "HDMI 2.1" certified doesn’t actually mean anything. A lot of these 2.0/2.1/2.2 cables really aren’t any better than older HDMI 1.3/1.4 cables in a lot of situations. Most will pass 4K, but it doesn’t mean anything about quality. I know of cables from 2007 (such as Wire World Starlight 5-2) that will also support 4K in the 1-5 meter length without anything special, but these cables provide a superior audio/video quality.

The best HDMI cable for the money I have found is Nordost Heimdall 2 with its 25awg solid core conductors (at a minimum 2 meter length). The older Wire World Starlight 5-2 or 6 cables are readily available. These 5-2/6 cables have a larger 24awg conductor, but lower grade copper and you -might- get some brightness/harshness from them (probably not with Marantz). However, they have better resolution and more "oomph" in the bass/midbass area.

Always get minimum 2 meter HDMI cables (even if the components are close). Short 1 meter type cables have problems with high frequency response and video sharpness due signal reflections on such a short cable.

The PS5 is also an internal switching power supply.  I don't know how good it is for disc playback.  Most bluray players nowadays are also switching power supply (so is Oppo).  You could try to get an Oppo 203 and then upgrade the power supply to linear (from oppomod website).

Or buy a Panasonic DP-UB9000 for $1,000 and then get the Oppomod upgrade for another $1,000.  That's a $2k investment, but I think that's as good as it gets for bluray players.

Though, most people will not go to this level and would be happy with a $200 disc player.