My assertion that the stereo mix is preferable for two-channel systems is more of a strong recommendation than an absolute “NO YOU CAN’T DO THAT!” sort of thing. 😉
In the next version (assuming again that I have time to do a next version), I’m hoping that this downmixing problem will be much less of an issue.
If you’re upscaling to 5.1 just for the purpose of including the LFE, I wouldn’t bother as it’s a lot more trouble than it’s worth. Maybe mix in the LFE @ -10dB.
Or there’s a more elegant way of delivering stereo+LFE - direct bitstream configuration.
AC3 & DTSMA can support 2.1 natively, and encoders are readily available.