Upon an upgrade to Ubuntu 15.04, both gtkpod and Banshee successfully sync my iPod Classic 160GB 6th gen (2nd rev), including cover art.
sudo apt-get install gtkpod
I recommend using Banshee to sync music, it has a much nicer interface and is much more stable than the
gtkpod GUI, but it uses
gtkpod in the background.
It may not be necessary to upgrade to 15.04 to do this, perhaps it would be possible to just compile the latest version of
gtkpod from here (currently on version 2.1.5) to fix the problem, but I haven't tested this yet.