Here's the "definitive" layout.
I scratched the pot, moved things around to get more room, added 2 more caps in the analog supply, made all the power distribution for the headamp by wires to avoid big power traces all over the board, added 2 pads to get the signal before the headamp.
Edit : power consumption problem :
This thing should eat at least 40mA for the DAC and Sijosae claims a 20mA consumption for the amp section. So, it gives us a 60mA total power consumption. On two rechargeable 9V batteries, with 200mA, it gives us about 6 hours and half of life. On 8 AA rechargeable batteries, at 2000mA, it gives us a more satisfactory 33 hours life
Another option would be to use the power the USB could give us. It'd take a 5 to 9V dc-dc converter and quite a bit of regulation to clean the power coming from the computer.