[LAU] On the fly bass downmixing to mono (mpd or alsa)?

[Matt Garman]

On Mon, Feb 10, 2014 at 10:52 AM, Joe Hartley <jh at brainiac.com> wrote:
> This becomes a matter of a simple Y-adapter to combine the two outputs
> into one.  Don't worry at all about filtering out the signals at the
> server end, the subwoofer will have a low-pass filter that will cut out
> anything above a certain frequency anyway.  On some, the frequency of
> the low-pass filter is adjustable.

I'm not sure this is right.  Based on what I was able to glean from
searching the web, there are a couple issues with this:
    (1) Depending on how the DAC outputs are wired, using a Y-adapter
on one set of stereo outputs might also cause the other set of stereo
outputs to be mono as well.
    (2) At least a basic resistor network (one has already been
described in this thread) is needed to "correctly" sum the two
channels.  Now I can't find the link that gave an explanation of why
this is, but my takeaway was that a simple Y-adapter isn't the way to
go.