Is this the only treatment you plan? Unless you already have these diffusors I think you would be better investing the money in absorbent panels. In fact it would be better still if you understand a bit about what you are trying to achieve by using room treatment (forgive me if you already do!) before investing in it. It's a complicated business and it's easy to waste money for little result, or the wrong results.
You are right when you say that diffusers need a certain distance to work properly - too close and you can hear them as I discovered early on my journey!
I only understand stereo so I can't properly advise. The methods of controlling the sound in your room is:
1. Careful positioning of speakers and listening position.
2. Room treatment (bass traps, lighter panels for reflections, and perhaps diffusers).
3. Sub woofers - one or more carefully positioned.
3. DSP/EQ - to tidy things up.
We on here recommend measuring with a microphone and software to understand fully what is needed and what we are doing. You could just use your ears if you can't be bothered with all this but quite frankly I didn't know good sound until I measured it and got used to it. For example, I thought I had good bass but in fact it was just some frequencies that were loud (boomy really) and once you get each bass frequency at an equal level, the whole musical spectrum opens up - you get musical bass.
Of course what you can do is decided by finance and your domestic situation so you may not be able to do all these.
Most important is getting bass right. Subs and/or DSP/EQ can do this alone although room treatment is better as it helps allot with decay times too - giving a tighter bass.
Next deal with side wall and other reflections. Some people like these - I don't as I find stopping reflections gives a sharper image and sound.
Go just over half way down the linked page and there's a film called 'All About Diffusion':
https://realtraps.com/videos.htm
Realtraps has lots of info including some videos and is worth exploring.
