ipod classic distrotion

Hi - I have a ipod classic 160gb 6th generation. I've always been really pleased with the sound. I haven't imported new CDs to itunes for about 6 months, but after downloading itunes 12 last month, I have imported a lot of CDs only to find that the playback on all are really distorted through headphones when playing my ipod (they seem to be ok on my computer and on my ipad). I have checked the headphones and jack and all are fine. I have also changed the soundcheck and EQ and this doesnt help. Is itunes 12 compatible with my old ipod? Is there anything I can do?