Mhhhmm yummie. I want one.
Decibel is a logarithmic "helper unit" that describes a ratio of power or field quantities (such as sound pressure, voltages etc.).
Logarithmic means that even huge ratios can be described with relatively small decibel numbers.
You put 1V into an amp and measure a max output of 5V:
5V / 1V = gain of 5x
20*log10(5) = gain of +14 dB
You put 1V into an amp and the max output is 0.1V (a tenth):
0.1/1 = gain of 0.1x
20*log10(0.1) = gain of -20 dB
As you can see, as soon as the gain factor is lower than 1x you get negative dB. For power you need to use 10*log10(ratio).
Now if some specs say 20 Hz - 20 kHz +/- 3 dB this means that in the whole range the frequency response can vary by 3 dB up or down.
+/- 10 dB is perceived as twice/half as loud.
It gets more confusing since above numbers are completely relative, sometimes written as dBr. There are some common dB units that have a reference, such as dB SPL (reference is a sound pressure of 20 µPa), dBV (reference is 1 V) etc.
If you know the headphones' and cable's impedance at 1 kHz you can calculate it like a voltage divider.
For example 20 ohm headphone, 5 ohm cable1, 3 ohm cable2, all at 1 kHz:
20/(20+5) = 0.8
20/(20+3) = 0.8696
20*log10(0.8696/0.8) = +0.72 dB for cable2
You could also measure the voltage across one headphone driver with the cable connected. (Gonna need a little self-built adapter for that so you have access with the probes.) For 1V output from the amp you should measure the same numbers as calculated above, assuming the amp has 0 ohm output impedance.
Unless there is some extremely well-implemented noise shaping and filtering going on with both the ADC (recording) and DAC (reproduction), DSD is measurably inferior to even 24/96.
Some would even say worse then 24/48.