01) -6 dB is fine for mixing, but probably too low for customers. Do some sort of mastering before you give it to someone to listen, like Magnus recommended.
02) Take time to switch off every single plugin in Logic and see which one creates the hiss. Sometimes it's only one, sometimes it's the accumulation of several plugins.
Look especially for plugins that simulate hardware. Maybe you left an amp emulation running without a gate? If you found it, please tell, I'm curious.