A majority of the time customers with this complaint are using headphones that have low impedance (Less than 25ohms). This will cause the headphones to be sensitive to weak amplifiers (like portable devices and phones). The users comfortable listening level will be low on the SUBPAC's input, causing the intensity level to increase to get the right feel. Switching to a higher impedance headphone (no more than 32ohms) will help balance this by lowering the sensitivity of the headphone so it can take more level.
Connecting sensitive low impedance headphones to an amplified output (like a DJ mixer or audio interface) can "blow" them out. Connecting impedances higher than 32ohms to the SUBPAC will still work, but with diminishing returns in performance as the number increases.
Impedance is a good guide but doesn't always correlate with sensitivity so its best to try a few different source devices and headphone combinations. Results can vary from subtle to drastic. You can also use a headphone attenuator to help balance the system.
If the self noise is intolerable another solution is to monitor the headphones from a separate, dedicated headphone amp that shares/parallels the signal sent to the SUBPAC. In a mobile situation, the noise can be eliminated using a y splitter in two ways. 1. Plug the male end into the source device and the SUBPAC and headphones into the female ends.
You will need a splitter like this. http://www.amazon.com/gp/product/B000067RC4/ref=s9_simh_gw_p23_d0_i1?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=desktop-1&pf_rd_r=0VF0EA6YKV7KAJM07VHW&pf_rd_t=36701&pf_rd_p=2079475242&pf_rd_i=desktop