I have the Zoom G2.1Nu effects pedal. It has a 6.3mm mono out. I use a stereo adapter 6.3mm-to-3.5mm jack. Pedal is connected to the Mac via USB so the Mac's audio output is going through it.
I have a 2.1 sound system with some... ummm... control unit (here it is) and headphones.
So the question described below.
- Sound system (about $120) playing stereo sound from guitar (which has mono out), as well as from Mac.
- Some very cheap (about $10) headphones and sound systems also play stereo sound.
- Headphones (about $30) playing mono sound to the right channel from both sources even when connected to the 3.5mm out on any of those sound systems (though they are 100% working with stereo output from iPhone or Mac directly).