You are using an attenuating cable to make it so that the headphone signal output of the H4n will drop to the line input level your DSLR expects, however you are confusing gain and sensitivity. Gain is an analog means of boosting a signal. It is increasing a signal beyond 0dB signal and uses something called a pre-amp to increase the signal strength.

Instead, you have turned down the sensitivity of the 6D thinking you were telling it not to apply gain to the signal, but instead you are cutting all the signal strength. Another possibility is that you may have read feedback from people who were trying to use a non-attenuating cable to insert a line level signal direct in to a mic level jack. This produces low quality and would require a very minimal sensitivity to try to salvage the signal.

The attenuation cable you are using is the much better quality way to deal with the impedance mismatch and you should simply turn up the sensitivity on the 6d to a normal level, which should give you a good signal level on the 6D.

The main things you want to avoid to maintain signal quality is ensure you have a strong signal at every part of the audio chain, but you don't want to ever clip and you don't want impedance mismatches and you don't want to artificially gain if you can avoid it. The attenuation cable counteracts the impedance mismatch, you should send out of the H4n no higher than 0db, but you should only reduce the level if it is resulting in clipping or having to turn down the sensitivity on the 6D too much. Then you should set the sensitivity on the 6D to avoid having clipping issues and ensure strong signal strength.

You are using an attenuating cable to make it so that the headphone signal output of the H4n will drop to the line input level your DSLR expects, however you are confusing gain and sensitivity. Gain is an analog means of boosting a signal. It is increasing a signal beyond 0dB signal and uses something called a pre-amp to increase the signal strength.

Instead, you have turned down the sensitivity of the 6D thinking you were telling it not to apply gain to the signal, but instead you are cutting all the signal strength. Another possibility is that you may have read feedback from people who were trying to use a non-attenuating cable to insert a line level signal direct in to a mic level jack. This produces low quality and would require a very minimal sensitivity to try to salvage the signal.

The attenuation cable you are using is the much better quality way to deal with the impedance mismatch and you should simply turn up the sensitivity on the 6d to a normal level, which should give you a good signal level on the 6D.

The main things you want to avoid to maintain signal quality is ensure you have a strong signal at every part of the audio chain, but you don't want to ever clip and you don't want impedance mismatches and you don't want to artificially gain if you can avoid it. The attenuation cable counteracts the impedance mismatch, you should send out of the H4n no higher than 0db, but you should only reduce the level if it is resulting in clipping or having to turn down the sensitivity on the 6D too much. Then you should set the sensitivity on the 6D to avoid having clipping issues and ensure strong signal strength.

Update: I may be partially incorrect on how the camera actually processes things as OwenM pointed out that ML reveals that it appears there is a pre-amp in the camera. That said, Having the level all the way down is still applying a reduction in signal strength or sensitivity that you don't want. You just don't want to be applying unnecessary gain to the signal (which would also amplify any noise and introduce some new noise.)