How exactly does one know that the "qualified installer" did a "proper" job, without going right back in behind them and checking the work??

You're usually going on faith, because all will be hidden, which isn't something you should be forced to do, just because some wonker didn't get your car right when you ordered it new.

There are definitely good audio shops out there. But I wouldn't have work like that done, unless the proprietor was willing to allow me to spend the day overseeing the work as it's done.

Call me anal. I know I am. I also don't allow tires to be mounted without being right on their shouder. Last time I had new Michelins mounted to my magnesium Speedlines on my Saleen, I wound up doing all four tires myself, using their equipment, to show the dolts how to do it without risking damage. Needless to say, they didn't charge me for the mount & balance. But I digress....