Headphone loudness calculator with DIY PCM1794A DAC

These few months I was playing with my DIY PCM1794A DAC. I was using the output schematic specified in the datasheet PG. 25:

After hooking up the circuit to my headphone (ATH-CKS1000), I was found the sound level was incredibly loud (not enough to damage my hearing/headphone though..), with some calculations, I found out the reason:

1. Note that the output is 2Vrms, U1/U2 is formed as an active I/V, converting IOUT-, IOUT+ to voltage. 
2. At -FS, IOUT- gives out -2.3mA, IOUT+ gives out -10.1mA; At +FS, IOUT- gives out -10.1mA, IOUT+ gives out -2.3mA
3. So for example, at -FS, U1 will gives out 750Ohm*(-2.3mA)=-1.725V, and U2 will gives out 750Ohm*(-10.1mA)=-7.575V
4. You can refer to the following table for more details:
   
5. U3 is configured as a differential amplifier, using -FS as an example again, U1 output -1.725V; U2 output -7.575V; Output of U3 = (270R/560R)*((-7.575V)-(-1.725V))≈-2.82V, formula:
   
6. You can also confirm the findings from the above table.
7. Output is ±2.821V, which is ≈1.995Vrms 
8. Notice there is R7 (100Ohm) at the output, for my 16Ohm headphone connected in series with it, the output stage turns into a voltage divider network:
9. So my headphone will receive MAX 2.821V * (16/(16+100))  ≈0.3891V
10. Power delivered to my headphone: 0.3891^2/16=9.462mW, which is less than the maximum input power 100mW. But don't get fooled by this! 9.462mW is already REALLY LOUD, you do not need the full 100mW!!
11. The sensitivity of my headphone is 106dB/mW, so for 9.462mW, the actual output loudness is: 106+10*log(9.462)≈115.76dB SPL
12. Taking the dB SPL chart from here, you can discover that 116dB SPL is already quite loud:
    

I have also made a small calculator here, you can try out different values:

Audio System Output
Vrms out
Series resistor (R7)/Total output impedance
Headphone
Impedance
Sensitivity (dB/mW)