[jjd-uk]

Aha, interesting!
Lovely reply, it gave me alot of useful tidbits =)

I'm a bit confused though, I thought bitstreaming was equivalent with pass-through. In that case the computer feeding the source material to the AVR shouldn't really matter, as long as the HDMI cable supports it, right?
I can imagine most users use the built-in soundcard on their motherboard to pass the audio along. I am, anyway, and by looking at the properties for the audio device in Windows I can see a bunch of formats available under the Supported Formats section. Even Dolby TrueHD - which is making this even weirder. I'd dare rule out the HDMI cable being the culprit in that case.
Granted, the laptop is quite old but I'll try with a newer one tonight (along with a new HDMI cable to be sure) and report back. Thanks for the quick replies guys <3

If Windows lists TrueHD then your problem is most likely this:

If you don't have Atmos in the display it could be a wrong receiver configuration
or your hardware is not capable to bitstream or a wrong configuration in your hardware/software.

Since Atmos is nothing more than a TrueHD stream with some extensions then the AVR needs to be told how to handle it, so in the AVR configuration there will usually be something to tell the AVR whether to decode as a normal TrueHD stream or as Atmos.