Thanks for the quick reply tomb - I connected the other 680uf cap to the DC jack and that also reduced the hum/noise - tried connecting the 220uf caps also but that increased the noise. So I disconnected the 220uf and reconnected them to the PCB ground plane and then put everything back into...

I built a SSMH several years ago, but never used it much because all of my headphones at the time were low impedance - but recently got a pair of 250 ohm DT990 and none of my DAP's could really drive them well enough so I pulled out the SSMH - there was too much noise and hum so I decided to...