With the in-ceiling, so long as you place them somewhere close to Dolby's recommendations, the AV Receiver will sort this side out for you
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As for the type of speakers, as it isn't as important to match Atmos speakers compared to the front three, the only advice I would give is to look at the largest ones available to you. These will usually be 8" size wise
The front three, Use LCR an orientate them all the same way so the sound is balanced across the front three and as you are looking to use a JVC, ensure you take note of which HDMI cable others are using for the distance you are looking to run yours. If you get it wrong, it will cost a small fortune
For the AV Receiver. I would say, try and jump to the X6700H as a minimum as it has a few extra bits which should be more important to you like (Mono Block Power Amp Construction) plus alpha processing if required
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It's a big jump in price, but it will be worth it sonically as sound makes up just as much of the experience of what the projector does
As the JVC has memory zoom, maybe also think about a 2.35:1 screen option. It's easier to taking in a wider screen which is about 33% larger than a similar sized screen in a 16/9 format. So, when working out the throw for a 1.78:1 screen, if you use the memory zoom with the JVC, this would require you to place the projector between these measurements 4.10m to 6.50m whilst enabling you to use a screen which is 125" diagonal 49" height and 115" wide. This would also maintain the 100" diagonal aspect
If you look at a like the Grandview Cyber which is a couple of inches wider than the guided throw I'm mentioned above, then it will be somewhere close to those measurement, but will need recalculating if close. These screens are not the cheapest fixed screen, but if you are using this option, then ensuring the fabric on both the screen and surround does it job correctly will really assist with any light leakage. The other thing to note when using this option, be careful with reflective objects close to the screen as the JVC doesn't change its aspect ratio, it only fills the screen size using its memory zoom
Now I'm going to push your budget on the front three speakers as they will be the most important and once they're in by suggesting either the Dali Phantom M-250 or the Focal 300 IWLCR6 in walls for the LCR's
With the rear's, are they wall or in-wall?