The build quality of the box isn't the issue; it's just that a pre-fab bandpass enclosure rarely matches up well with a random subwoofer. Decent sounding bandpass systems are usually designed with a specific speaker in mind.
he is just trying to make a sale, all he wants is the cash and thinks that you will fall for it. post everything here and get feedback from us and we give you opinions and let you know what you should and shouldnt buy