iTunes no longer plays song files in 5.1 - only in Stereo

Greetings!

Unfortunately, since a few days ago, iTunes has stopped playing my songs in surround sound and is instead only using my front speakers currently, while I can play the very same songs in surround sound using a different audio player. I've tried the following to no avail (in no particular order):

• rebooted
• completely reinstalled iTunes
• used a System Restore Point from before the problem occured
• reinstalled the driver for my Sound card (Asus Xonar DSX)
• set Speakers to 5.1 in in both the Xonar DSX Audio Center and under Windows/Playback/Speakers/Configure
• disabled my Realtek HD Audio device in my BIOS
• installed Quicktime and set Audio to 5.1 under Preferences
• switched between Direct Sound/Windows Audio Session in iTunes' Preferences/Playback
• removed and reinstalled my Audio devices in the Device Manager
• plugged in/out my USB Headset (which I think might've been the cause of the problem, since it usually causes iTunes to go into Stereo mode until I restart the program)

My specs are:

• Windows 7 64-bit
• iTunes 11.0.2.26
• Asus Xonar DSX 7.1 Surround, latest driver
• Teufel 5.1 Surround system (all speakers still work perfectly)

Sadly, nothing helped. And I'm sure iTunes can play the songs in surround, since it has been doing so ever since I first started using it (and it has been my go-to program to test my audio for troubleshooting up until now).

Thanks for any replies!