the mach 460 sounds pretty bad. aac is lossless compression so it doesnt sound any worse becasue its coming off my ipod. the mp3 i rip are all at least 256k. so by not using fm and hardwiring it into the head unit, it sounds only as bad as the mach 460 makes it sound. fm sounds a whole lot worse.
#1 rule of audio - tamper with the signal as little as possible.