Hi, new on hear, hope you can help. All i want to do is play two keyboards through one pair of headphones, but one keyboard is allways very quiet whilst other is fine. I am plugging in a splitter cable to the headphone outlets of both keyboards. namely a 2x 6.5mm stereo jacks to 1x 6.5mm stereo socket. The jacks into the synths and the socket i plug in headphones. Do i need really good quality cables, or some kind of simple headphone amp etc. Any help would be great thanks.