iOS5 seems to have introduced distortion on music

I updated both my iPad and iPhone 4 to iOS5 and when I listen to music through headphones and earphones on both, there is often distortion when playing music. The headphones are B+W and the earphones are also high quality. I have tested the phones on Hi-Fi equipment, which would produce a bigger test and there are no problems. There was no issue with the sound on either the iPad or iPhone before I upgraded. I restored the iPad and the problem is still there.
If I stream music from my iMac to the iPad the problem isn't there!
Does anyone know if this is a problem in iOS5, or does anyone know how to solve this issue?
Both devices are now no use for listening to music on and I have reverted to my iPod classic