Who said what again? Ha! Edwin, I only know enough about electronics to get myself into trouble ... but I believe Elwood is correct in that the issue here is an impedance mismatch.
My understanding is that the output impedance of any piece of gear wants to see an equal or greater input impedance of the next piece in the chain for things to be happy. And headphones, like speakers, are generally very low impedance so it's very close to shorting the output.
I've never known the actual output impedance of the Series instruments, and it may vary because of the master volume, trimpots, mono summing resistors, in-line pads, etc. (Told you I don't know what I'm doing.)
All that said, Greg is recalling a story I told about an LA studio with a terrible RF hum problem. In that case I plugged a pair of phones into the 1/4" jack, turned up the volume, and walked around the building trying to find the quietest spot. I run my preamp boards cranked so despite the impedance mismatch there was enough sound available for me to hear the hum.
I have often driven power amps directly from the bass and on a standard bass amps will try going directly into the FX return. I love that super clean and fast sound. My Walter Woods is a different thing in that it has a built-in 5-pin jack which powers the bass directly from the amps power supply. But the audio signal there is padded down and run into the channel inputs... blah blah blah.
In conclusion... I think impedance is the winning answer!
Jimmy J
