PC graphic card HDMI out -> AV Receiver -> headphones , how much of a bad idea it is?

I think this is perfectly legit. HDMI in an audio sense is even superior to S/PDIF (more bandwidth), but it does the same job: carry a digital signal. i.e. This is not a bad idea at all. In the end it's just like USB or S/PDIF.

That depends on whether the receiver has any sort of virtual surround processing like Dolby Headphone, and also whether or not you care for hardware sound acceleration in older PC games designed around sound card DSPs, which modern graphics cards with HDMI output will not suitably accommodate. (Not even basic EAX 1/2 support, which even Realtek codecs provide.)

If neither of these bother you, then it's a good solution.

Quote:

Originally Posted by madmalkav 

I have seen an offer on a Marantz NR1403 and I feel curious about how this combination might work. I find it interesting as the Marantz will be a good upgrade for my current AV receiver, but I can't / want to buy a subwoofer right now -as my current subwoofer is integrated in the amp-. This way I can use it to power my DT770 600 ohms until I get some money for a subwoofer and a dedicated headphone amp.

It will certainly produce stereo without problem

The question, as NamelessFPG notes, is what happens at the receiver if the PC delivers multi-channel audio. I'm still a two-channel guy when it comes to headphones.

Quote:

Originally Posted by madmalkav 

No problem with old games -by the way, wouldn't those resort to some software processing if not DSP founds? We have seriously powerful CPUs now to compensate for that-, and sincerely doesn't have a single idea if this have some kind of Dolby Headphone like stuff -configuring output on 5.1 and testing the speakers gives different sound i.e. for front left vs rear left if that can be some indication-.

Games made with DirectSound3D and OpenAL in mind, back when CPUs were much slower and utilizing sound card DSPs was the way to go for maximum performance, generally have awful-sounding software fallbacks. Reverb/chorus/occlusion effects can go missing entirely, polyphony (amount of sounds that can be played simultaneously) is limited, etc. It's even worse under the post-Vista sound stack for DirectSound3D, where you'll be lucky to get even one-dimensional stereo sound without a DS3D-to-OAL wrapper.

OpenAL games have it easier with OpenAL Soft, Rapture3D, AeonWave, etc. to render sound in software and being unaffected by the Vista sound stack change, but they unfortunately don't have DirectSound3D wrappers of their own, limiting their usefulness for older games. (At least the more important ones like Unreal Tournament and Thief 1/2 now have native OpenAL patches, but that still leaves a lot of games stuck with the deprecated DS3D.)

Modern audio middleware like XAudio2/X3DAudio and FMOD Ex works out better than the usual DS3D/OAL software fallbacks, but is still noticeably inferior to hardware DS3D/OAL implementations (or A3D on Aureal Vortex cards, but that's old Win9x-era stuff), in no small part due to the lack of a binaural HRTF mixing option for us headphone users. Ideally, that would be built-in and we'd have positioning far beyond what Dolby Headphone could offer without having to buy particular hardware, but that's not how things turned out.