Dull sound from YouTube from TV plugged into hi-fi

[gmatkin]

We use the optical audio output of our UE32M5500AK  TV to feed our Denon RCD-M40DB (set to flat eq), which either powers a pair of passive 8in Behringer Truth monitors or a pair of AKG k702 headphones.

We also have a Google Chromecast 4K.

It should sound pretty good, I think, but I'm baffled by the sound quality we get from the system when used with YouTube. A case in point was this week when listened to a YouTube video we knew had a shaky egg on it - the shaky egg is clear on the same headphones when listening to the YouTube video via a phone, but disappears completely when playing on the Tv-hifi combo. It makes no difference when we listen via speakers or headphones, and is the same with either the Samsung YouTube app, or the Chromecast app.

It seems as if frequencies above maybe 6kHz are simply missing.

The Denon/Behringer gear seems to reproduce these frequencies as you'd expect when used with CDs, radio etc. So, what could be causing the problem please, and is there a fix? Is it a matter of wrong settings or perhaps signal compression?

Thanks for your help, Gavin Atkin 

[gmatkin]

Thanks for that.

I already had all the signal processing set to off (I should have mentioned it!), but following your suggestion this morning tried it again both ways. No dice, sadly.

Gavin 

[gmatkin]

I've listened to a YouTube that sweeps through the whole audio frequency range. As I thought, the signal drops out at 6-7kHz. I'm an older chap but in real life I can still hear shaky eggs clearly, and also when I play this YouTube on other devices.

So bearing in mid that I'm hearing this on both my speakers and headphones is this issue most likely to be down to the TV, the Denon, or the connection between?

Thanks, Gavin 

[gmatkin]

No - I'd be reluctant to do something like that, as I like to have eq set flat. In my (musician's) mind that's the sound that the engineers and artists worked on and heard, so I usually avoid boosts, cuts and other effects.  

But thanks for the thought.

Gavin