UR12 - Low quality output on headphones

Hi there - I am using Steinberg UR12 on Abbleton, it works great with active speakers an ASIO for input/output. However when I activate headphones output (i.e. headphones plugged in and speakers turned off), the sound is very poor quality. I have standard 3.5mm jack headphones with an adaptor to the 6.3mm Steinberg input. Any ideas as to why the output on headphones is low quality? Thanks in advance ! : )