No surround sound in quadrophonic setups (only stereo)

[VarHyid]

My PC is connected to an A/V receiver which is connected to 4 speakers - 2 in the front and 2 in the back. In Windows (8.1) I have the sound settings ("Playback devices") set up as "Quadrophonic" - works fine with every game, but in Warframe I'm only getting stereo sound (only front speakers are active).

This seems to be a bug only with quadrophony because when I switched to "5.1 Surround" it works as expected, all 4 speakers are active, sound positioning is correct etc.

That said, I can play like this because despite the lack of an actual center speaker and a separate sub-woofer, my receiver can "simulate" it so I do hear sound that's supposed to go to the front center speaker, it's just reproduced by the front L/R speakers, but still, it would be great if you'd fix this.