It's actually quite easy to change the resistors if you have soldering skills. Newer models have the 1Kohm or higher value resistor, Mine came with a 1,3Kohm resistor that is why it's under powered for my LCD. But when I changed it with a 270ohm resistor, Everything changes. 
My guess will be around the year 2005-2006 when they did the new reversion of the amp using the WBT binding post that's also when they changed the Headphone out resistors.






















