Play the songs you wanna play!If they are too hard, just use riff repeater to slow them down and then when you are comfortable, speed up a bit. Choose the coolest, most difficult bass lines you can think of and start nice and slow... Music is about feeling. Play the music that makes you feel good and when you are able to play it properly at the right speed, the rewards will be that much sweeter than if you played an old music just because it's easy enough for you. I suggest Hysteria and The Lemon Song ;)