Well, if you want to do it simply, then just search for Music files in a search window on the USB stick, then select them all cut and paste into the main USB drive itself not in any folder... and that should give you a list of all the songs right in the main directory.
The Other way would be to actuall have just one folder of either Albums/Classification/Artists/Genre/ etc etc
The Amp should be able to read upto a certain depth within the drive may not be 10 or 15 but upto 2 to three folders yes....PS: what amp are you using?