[Fungus]

Headphone specs: 
Impedance: 600 ohm
SPL: 120 dB 
 
Will the JSD Labs C5 amp power the headphone efficiently? 

Max Output @ 600Ω	4.146 VRMS​
Max Output @ 150Ω	3.580 VRMS​
Max Output @ 32Ω	1.182 VRMS​
Power Supply	14.0 Vpp​
Output Impedance	0.62 Ω​

 

[PleasantSounds]

If you're quoting the figures correctly, you need 1 mW to generate 120dB SPL, which is deafening.
The C5 can deliver about 30 mW at 600 Ohm. Draw your own conclusions...