I have an Erica Synths Pico Out, which is a small 3hp module that does exactly what you're asking about. It has a headphone jack with a volume knob and a separate line out jack without one. I think they're like $90. It's stereo, but if you only send one channel to the left input, it will output through both sides.
Simple but effective.
-- tessa
This for 3hp sounds great - so the XOH at 6hp kind of does the pico twice with the bonus of being able to mix both a and b inputs together + headphone/ouput has volume knob.
Plugging it in and having sound out of both speakers is something I am looking forward too.
I found a second hand XOH a little bit cheaper than a new Pico here in the UK.
Thank you for giving me another option.