I’d be very surprised if there was not a IC in there doing the heavy lifting. That said there is a lot more to coming up with a great design then just putting a chip on the board and following the reference. A great deal of work goes into getting all you can out of that chip. There is a lot of trail and error and trade offs which need to made ... and that’s just the amp board. Now you have support circuits like balance inputs along with the single ended ones and their interaction. Power supplies are a big deal. Problems like ripple on your supply caused by one amp may cause havoc in another. Keeping noise down Yada,yada,yada And then ya get into cases and case layout, testing to meet and pass government regulations ... documents on design, test, QA, build steps, user manuals ....
Wait did you say only a couple of years??
