Mono channel config yields really quiet playback

[esprit.tordu]

After not finding anything about this issue I am posting here about it.  I have one speaker at work hooked up to my receiver.  Therefore, I am trying to downmix the rockbox output to mono so that everything plays through the one speaker.  Unfortunately, when I do this it gets very quiet, less than half the loudness of stereo.  I can barely hear it.  I don't know why this would be.
I am using rockbox 3.5 on a sansa e280.  Everything works fine, even the mini sd card. 
Mono doesn't sound very good over headphones but it seems to be normal loudness at least which I guess isolates this as more of a receiver issue perhaps.  Anybody else have this problem?
Also I notice that if I unplug the line out cord halfway so it makes a connection with the other channel it works great!

[Multiplex]

maybe you are using a mono plug, that will be shorting out one of the channels and may be causing the output amplifier to struggle to drive the other channel. This seems likely as you say that if you pull the plug slightly it sounds louder.

Try making up a lead with a stereo plug and only connect one channel.