That’s a good question. I’ve never thought about that.
According to Dolby, the LFE channel should be dropped when doing downmixing.
http://www.dolby.com/uploadedFiles/Assets/US/Doc/Professional/18_Metadata.Guide.pdf
See page 12:
"In all downmixes, the LFE channel is not included. "
But that’s specific to Dolby Technology. I’m not going to read the whole thing to see what the implications are for other devices… :)
But if you Google the phrase “Does LFE belong in a stereo downmix” (without the quotes) you’ll see tons of spirited argument!
