my understanding is that you cannot copyright a "circuit" but you can copyright the "artwork". now, artwork in this case has a special meaning, its kind of a screwed up law, but the traces on a pcb are considered "artwork". so if you copied the pcb layout, you would be stealing the "artwork". if you took the schematic, made your own pcb that looked nothing like the original, then you would be fine.
my understanding is that the aesthetics are also under copyright, ie, the layout of the knobs, the names of the knobs ie "drive, pressure, honk" etc and the graphical design of "outside" of the device. the name of the original is obviously also under copyright.
my understanding is that it's not ok to say "this is an API2500 in a new box", but people skate very close to the line by saying "based on .." etc. or, they tell you its a "clone" behind closed doors. or, its just "known" thats its a clone and people talk about it on forums.
this is why you see identical circuits, in what appear to be completely different products.