As soon as I have the electronics, I will focus on the room. I will have to buy something anyway, before I fry my speakers with my old Yamaha RXV-440RDS AVR (way below 60% of the KEF's power rating: https://ca.kef.com/blogs/news/recommended-amplifier-power), since I like to crank it up quite a lot during action movies.It is very very likely that the separate chain will not improve sound in its own as speakers, room and room corrections affect sound by orders of magnitude more than all the electronics together.
Plus “better” is very undefined and in your case (and many others) it is simply what you like most.
To get there simply use one of the recommended room correction system and spend some time with it. There is no silver bullet system chain , which ensures that you like the result. And you actually adjusting the room correction system until you like the result is by far more effective, than (randomly) changing electronic components, which are measurably different but audibly are not distinguishable.
Power amps: Start with the X4800 and if you find that (by measuring) you want to listen very loud and have problems with your speaker impedance you can add a power amp later (I have two mono blocks hooked up to my AVR, driving my KEF Reference 3, but believe me, I am very very hard pressed to hear a difference).
Music and HT: I use two settings, in the room correction system (in my case Audyssey with MultiEQX) one for Music and one for HT. All one needs. (Yes I do have a second system in my office. But that can come later).
I'm just curious if - during critical music listening - I can hear any audible improvements with a better quality multichannel DAC like the Okto DAC8 Pro or the DM7 plus Dirac without downsampling, compared to the x4800h. The problem is that I look at the signature (list of devices) of other members too often and I can see many people running a Mac or PC based 2.2 system (in their living room!). That's how I figured that there must be a reason behind. I have this idea that if I can squeeze better performance/quality from the KEFs, why not. I've also made the mistake of reading the KEF Ref3 review on soundnews and it mentions that this speaker can scale with more powerful amps and better quality components.
As mentioned, HT is secondary for me, I just want a nice immersive experience during the occasional movie evenings, with plenty of grunt. This is why I'll add my old ported sub as a third one to the 2 sealed that I will use for music. For this secondary purpose the x4800h is very good and comes with a very good price as well here in Europe, recently below 1200EUR. Even an x3800h would probably do the job since it has all the features that I need. The output voltage is a bit low for the NCx500 (it would have to be used with the highest gain setting), but the Purifi 1ET7040 seems to be a good pairing.
Between a properly set up Audyssey MultEQ-X and DLBC system I don't expect much difference either. Especially if I set the curtain to 500Hz or lower. But I don't have the option to use Audyssey on the 2.2 chain, it requires an AVR that must be bought in advance, even before you buy the MultEQ-X licence.
No audible difference on an Auralic DAC? May I ask what is your current setup / how you drive the KEFs?I think you have most of the pros and cons. And after all. Your money, your decision. And maybe you have to go down the separate chain rabbit hole to find out yourself. I have been there (Dedicated Auralic DAC) and I built it back. Have fun.
I am starting from scratch (I have only the speakers, plus the trial DLBC licence), so it's not too late, I just need a push to the right direction. I'm a perfectionist and it makes it extremely hard to make the right decision in an area where I'm not yet knowledgable and you're surrounded with subjective opinions.
Anyways, I appreciate all of your advice!