Thank you for all that info and research for me! I'll answer your questions first:
What's your source that your trying to play? UHD Disc or Netflix, Disney, HBO? Also which HDTV are you using? Is your Disc Player connected to your HDTV or the Soundbar? Also Does your HDTV have ARC or eARC? Also does your HDTV support Dolby Atmos? And finally do the HDMI cables your using support 48Gbps high speed & eARC?
The Dynamic Range Compression I'm experiencing occurs on everything - UHD Disc, Streaming, Sky etc.
TV is LG C9PLA, which does have eARC capability but the soundbar doesn't.
My UHD Disc player has 2 HDMI outputs, one video and one audio which is going direct into the soundbar.
The TV itself does support Dolby Atmos (and claims to be able to play it through the in-built speakers).
Yes I use the Ultra High Speed HDMI cables.
The thing is, it's not a case of the difference between lossy / lossless audio, because frankly unless you're an audiophile we're talking about pretty subtle differences there. The issue I'm referring to with dynamic range compression is glaringly obvious to anyone - eg. you'll have a quiet spoken dialogue moment be considerably louder than an explosion seconds later, which suddenly sounds squashed like there's a limiter.
Another clear indicator something fishy is going on is the fact that with every film using DTS-HD Master HD / DTS:X / Dolby TrueHD / Dolby Digital, the volume always needs to be set at 20 minimum (and higher depending on mix). When in Dolby Atmos mode, the volume is always about 15, which is considerably lower yet produces the same overall level volume.
Taking this example even further, using my disc player I can set it so it chooses to play Dolby Digital instead of True HD / Atmos. The same film will require Dolby Digital set on volume 20, and then Atmos on volume 15.
To me none of this makes sense considering Atmos is built on Dolby TrueHD tracks. I don't know if it's a case of these LG soundbars simply translate the Atmos metadata with a weird dynamic range compression built into it.
I considered maybe Dolby Atmos metadata is too much for a soundbar to play naturally, so a limiter is required so it can cope. But the DTS equivalent (DTS:X) has no such issues, despite working the exact same way, and sounds fantastic with not a hint of dynamic range compression in sight!
To be honest I don't expect to find a solution for this, I'm much more curious if anyone else who has one of these soundbars has noticed this issue. I can't be the only one!