1. 5003 no ipod, USB interface
2. 5003 output level 2.25V, 6003 output 2.35V, you will feel 6003 more punch and force.
3. 6003 mechanical (chassics) better than 5003.
You can buy 5003, change the following items, will 4 times better than 6003.
1. Full mod capacitor, no need blackgate. Budget dependent
2. Up-sampling converter to 24/192 then feed it back to internal DAC.
3. Dampling material.
For detail, please join KK CD modification class 2010.