This has come up in other Toyotas - some have a three-level setting for ASL, some are just ON/OFF kind of thing. Only a few have noticed any difference in sound volume. It is supposed to change with vehicle speed - but only a few can tell that it works. Same deal with RDS on some - some get the information, some do not. My Matrix works just fine with RDS but lacks the ASL option - other car that I used to drive had it - I don\'t miss it at all.
Changer has two modes, like you mentioned, random and shuffle playing. In shuffle mode - it will switch discs - but you must set it to that mode. Many units by default will shuffle on the same disc (basically making it a random play mode rather than shuffle) - not sure how to enable that on the JBL system in the car.
Granted the JBL headunit is a nice unit - but experience has taught me to never buy an upgraded stereo system in the car if I have any chance of upgrading the system at a later date. Many times there are inline filters and components that tie the headunit to the speakers (tuned for each other). Replacing the system down the road without pulling up all the wiring may cause undesireable results (ie. new headunit = distorted sound through existing speakers due to tuned filtering). You are asking a lot from the headunit - Toyota tends to put the better stuff in Lexus models, for Toyota high end models - kind of hit or miss.