18dB is not needed at al. It is waaaayyyy too much.
That would make it a basshead headphone !
+7dB is more than enough otherwise the HD650 starts to sound bloated and 'fat'.
When the EQ is done digital and 32 bits is used with a 24 bit DAC you don't loose anything.
The effective dynamic range when listening to music in your livingroom (or with headphones) is about 70dB.
Assuming you would really want to boost 18dB at 20Hz and use a simple 6dB/octave filter (RC) use the filter described below.
I calculated a passive attenuator for you to try (inline with the headphone with 18dB 'boost' at 20Hz.
Total attenuation is 20dB so you need to connect it to a speaker amp output terminals to try.
Assuming you would play at max.1V then you would need a swing of 10V to reach that.
I won't publish a schematic as this would find its way and damage my 'reputation'.
The filter below is only for illustrative purposes about +18dB bass boost.
Also ... use music material that actually contains frequencies like this, say a large church organ.
1 resistor of 120 Ohm between the output + of an amplifier and the plug ring (and left channel tip) of the HD6XX One of the - connectors (assuming the amp is not balanced) to the common of the headphone plug.
+ in parallel to the headphone (so from ring to common and also one from tip to common a series circuit consisting of a 12 Ohm resistor + 47uF capacitor.
Should you want to try the 7dB boost simply change the 12 Ohm for 47 Ohm.
Tip: use a 100 Ohm potmeter and make it stepless, dial it in and measure the resistance to know how much boost is needed.
Nothing beats some good experimentation, trial and error and your own experiences.
note: if that was the only EQ it needed and it would be a simple passive solution built into an extension cord then I would have made those cords and have sold hundreds by now.