That was a recent aquisition that I haven't tinkered with. You have likely opened up a pandora's box by telling me there are jumpers I can play with. It sounds pretty damn amazing how it is. I got it off of Craigslist for $125.
It makes a difference with high impedance headphones and planars.
The direct drive mode which is engaged by the jumpers are its claim to fame. It does increase the noise floor, so there is a slight hiss, but it also unleashes the full power of that amp. The direct drive mode (which uses the jumpers that are included) has these specs
8 Ohms: 50 watts / channel
33 Ohms: 12 watts / channel
47 Ohms: 8.5 watts / channel
150 Ohms: 2.6 watts / channel
300 Ohms: 1.3 watts / channel
600 Ohms: 0.6 watts / channel
So your HD 6xx (150 ohms) and the HD600 (300 ohms) have HUGE amounts of power on tap.
They should also sound great with your planar magnetic fostex which is rated for a max input power of 3000 mwatts. Lots of punch and power.