yes it does.And be careful,the headphones are very loud,you should decrease the volume before you connect te headphones. It has almost blown my brain away.very bad experience.
Yeah, I learnt that the hard way !

I meant, in CnMC's particular case, does it turn off
his speakers ? If it doesn't, then it would suggest to me that there's a physical problem with the headphone socket not gripping the headphone plug quite tightly enoughto break the connection to the speakers. I'd be tempted to dismantle the machine and look at what the socket contactors are doing while you plug in your 'phones.
Alternatively it might be something as simple as the contact point on the common line has just become dirty and needs a bit of a clean up. A multimeter and a stereo 3.5mm plug would be a good help for diagnosing this problem.
Finally, sorry to ask obvious questions, but are you sure you're using a 3-conductor 3.5mm plug ? Some MP3 players use 2.5mm plugs and some iPods use a 4-conductor setup, so if you tried to use them on the LLL, you probably wouldn't hear anything. I had a portable CD player which had a 'remote control' on the headphone lead. The connector wouldn't work with any other equipment and the CD player wouldn't work with any other 'phones.