Comment 6 for bug 591036

[22:51:36] <crimsun_> JontheEchidna: RE 591036, you have a problematic toggle called 'Independent HP',0
[22:52:09] <crimsun_> JontheEchidna: depending on the version of Kubuntu (well, linux/alsa-driver), this toggle behaves in opposing manners
[22:52:49] <crimsun_> JontheEchidna: this toggle is tied to 'Master Front',0 and 'Front',0 both
[22:53:19] <crimsun_> JontheEchidna: so, you should experiment with all three and tell me which option gives you the "expected behaviour" -- whatever the heck that is :)
[23:08:50] <JontheEchidna> crimsun_: turns out, that I can get the effect I want by muting front with Independent HP at Off. I had been just trying setting front to zero, but that didn't work.

Now plugging/unplugging my headphones works as expected, with sound shifting between the internal speakers and the headphones appropriately.