If the physical size of a pre/pro or receiver will allow, as in the 990 that has internal real estate to spare, a manufacturer could leave a bay or two for special modules to be added to the otherwise completed system. That way, when someone wants a less-than-very-common input or other feature, the buyer can purchase an optional module to slide into the waiting bay. Other buyers are not saddled with the cost for all units to carry the feature. Of course, something that may have cost all buyers $50 or $100 will likely cost the occasional buyer four times as much.

So, we’re back to outboard add-ons where the buyer has a wider choice and, depending on the manufacturer, the benefit of a larger market for the special item and likely a lower cost than a special module suited only for a particular brand.