AVR HDMI ARC issue

[Dr Doofenshmirtz]

Ambleton said:

Whilst I agree with the sentiment, a good amp with decent speakers, set up properly with an optical source will always sound better than a poor system with the best source possible.

A small part of me thinks that a lot of it is "emperors new clothes".

a £2k av amp from 9 years ago purchased for £200 with an optical source will/does sound MUCH MUCH better than a new £200 ARC enabled 4k AVR.

I hear you...but we're getting into the realms of digital high definition audio now. So while you're listening to AES3 compressed audio at 48kHz on your older amp, all other things being equal, that £200 modern amp could be bathing your ears in Lossless DTS-HD at 192kHz (96kHz with 7.1 channels).
The modern amp will definitely be a more immersive surround sound listening experience. Having gone from optical to HDMI, even my untrained ears can tell the difference without really trying.
I would go so far as saying it's the difference between HD TV 1080p and 4K.