Just to check, you are measuring only the PC itself, not the monitor or any other device, right?
Your power supply is a bit old, but given a review I found, was about 80% efficient at low power levels, usually where any power supply is less efficient.
You did not mention your PC case, and any fans or add on cards you use also use power, although not much.
What are you using to check the CPU speed? Sorry, I don't understand "... powertop or on the states of CPU". That sounds like some kind of CPU monitoring program? How do you know, "Why c6 state missing?".
The only power saving feature options I see in your board's BIOS are Core C6 Mode, Package C6 Mode, and Cool 'n' Quiet. If they are all enabled, that is all you can do.
Windows Power Plans allow maximum and minimum processor states to be set, the minimum should be set below 10% for the most power savings. I don't know how Debian works in this respect.
Have you changed any CPU settings in the BIOS, like frequency or voltage?