Several years back I was doing some repairs on a couple alpine heads for friends and I happened to have their top 3 head units at the time together on the same bench. As I had them apart I started inspecting the boards and noticed that they were all the same, it just came down to what was left un-populated on the boards. The lower end had several components left off the board, and as you moved up in model number the boards became more populated (less components left off). Wish i could remember what the model numbers were. That was probably 10 years ago.