Talking about artifacts like fake L+R mixes being sent to the center channel...
Not sure if this applies, but for those using media player apps on the computer to play your surround music, some of these apps are shockingly buggy even here in 2015.
My last notes:
Songbird
pro
Set your OS audio utility to 5.1 (for speaker configuration) and all formats play out the correct speaker channels. (4.0 next to 5.1 next to 2.0 next to 5.0, etc, etc in the same playlist)
Plays FLAC files.
con
Last release (v2.2.0) has a bug that swaps channels 3,4 with 5,6 for uncompressed WAV files only. How embarrassing for them…
Apple lossless files play back screwed up with weird (and very obvious) distortion.
Play
pro
Plays all file formats including FLAC, WAV, OGG, SHN, Apple Lossless
con
You must set your OS audio utility to the specific channel format you intend to play (5.1 vs. 4.0 vs. 2.0, etc). That means no mixing 5.1 with 4.0, for example, in the same playlist. (Set to 5.1 and 4.0 program uses ch 1-4 instead of 1,2 & 5,6.)
iTunes
pro
None
con
Surround files do not play at all! You can't even drag them into a playlist. Just to add to their embarrassment, even their Apple Lossless format doesn't work! (It DOES support multichannel surround and works flawlessly in Play though.)
FLAC not supported (It's a format war thing.)
This app is hands down the most stunning failure!
Cog
Didn't really give this one much time. First impression wasn't good.
con
Crashes a lot.
Again, you must set your OS audio utility to the specific channel format you intend to play
VLC
pro
Supports most formats.
con
Again, you must set your OS audio utility to the specific channel format you intend to play.
4.0 quad played with a fake center channel L,R mix! Absolute show stopper there!!
Cannot play HD vidio + lossless discreet HD 5.1 audio (MKV format). Simply no audio output. It seems to only support lossy (encoded to 2 channels) surround typical in many movies.
Foobar
Didn't really give this one much time. First impression wasn't good.
con
No native OSX version last time I checked (but installs and seems to run without issue in OSX with Wine installed).
The "GUI" took me back to 1995!
XBMC Media Center
Note: This app is also a video player.
pro
Plays both the highest quality video formats and audio formats together including HD video with lossless discreet HD 5.1 audio. (MKV format)
con
The GUI is possibly the worst screen disaster I've ever seen. Looks like it was made by a 12 year old on copious amounts of Mountain Dew. Basic controls and functions are as clumsy as it gets!
Windows Media Player
Probably not even fair to review this. (I haven't actually even installed it in OSX.)
Hearsay
Doesn't support FLAC.
Might even only be an mp3 player.
None of them are perfect. Maybe some of the paid apps are - I don't know. I've been using Songbird because I like that I can mix and match formats and I keep everything in FLAC anyway.
And my point for this thread is: A fake center channel faux pas might not be the fault of your receiver. Your media player app could be doing it.