Yep. I have in my setup a Yamaha 7.2 surround sound receiver of ~ 2008 vintage driving 6 JBLs with an Infinity center and a Whafedale Sub and it Has "Enhanced 7 Channel or Enhanced 2 Channel Mode" that does just that and sounds good for music concerts where one would like room filling balanced Stereo instead of surround sound. Also has great Ambient Surround Sound Decoders (think 1980's Tate-Fosgate type stuff) as well as The Usual Dolby Digital and DTS decoders.Luckily most surround receivers has different modes which usually includes mirroring the front speakers to the back for stereo content.
What brought it all together as far as XBMC goes was getting a modern Quad Core 1080P native graphics capable laptop with hdmi out as this then works with XBMCs DTS and DD sound. The sound is awesome.
Streaming itself is the weakest link among all types of delivery systems but what really brought out XBMCs brilliance is when I use it to play backups of discs I own from my hard drive archives and the DD and DTS sound is fantastic. Considering that the only other real player with the kind of codecs to do this stuff justice is power DVD at $129, and XBMCs decoding is superior with never a sound video sync issue, the brilliance of XBMC is there to appreciate.
As to this thread.
If it were me, I would just get a Frys Electronics 5.1 SS Receiver for $159 and some quality used speakers via Craigslist and wire it up but that's just me as the surround sound is richer and better than any 2.1 Soundbar sub setup I have heard including a friend's high end setup in his home theater as nice as it does sound. Wish I had his home theater furniture though.....