I have separate 2-channel and 5.1 systems in different rooms. The 5.1 system in my basement is mainly for tv and movie watching while the 2.0 system in the living room is only for audio.
The question about 5.1 for audio vs. home theater reminded me of some of the competing recommendations for surround speaker placement for a 5.1 or 7.1 system under the Dolby guidelines compared with the recommendations for a purely audio-focused 5.1 system. There is a very interesting document produced by experts in audio recording/mixing working as part of a Grammy committee. (see this link: http://www2.grammy.com/pdfs/recording_academy/producers_and_engineers/5_1_rec.pdf )
I remember looking at these 5.1 speaker placement guidelines when setting up five ceiling-mounted Thiel PowerPoint speakers (plus a Thiel subwoofer) for my basement system. Thiel recommended placement of the two surround speakers farther back in the room than did the standard Dolby guidelines of placing the speakers more at the sides of the primary listening position (at 90-110 degrees, with the placement of the front middle speaker being considered the 0 degrees spot). The recommended placement by the Grammy group was at 135-150 degrees for the surround speakers in a 5.1 set-up for use by recording engineers when mixing 5-channel audio. This arrangement was similar to what Thiel was recommending, so I decided to follow those recommendations instead of the usual Dolby surround guidelines - and I’ve been happy with the results.
What I took away from these recommendations was that 5.1 optimized for audio recordings may be a little different than 5.1 for movie soundtracks - at least in terms of how sound is recorded and mixed. I don’t know if the recommendations of the Grammy group represent current practice, but the document they produced more than 15 years ago is still fascinating to read today.