You're not gonna like this, but...
RatShack sells inexpensive switchboxes that wil ldo exactly what you need. It will take one aux (high level) input on your receiver and allow you to use any one of four inputs for that one.
Usin' one on my Marantz 2270 for years now. Great unit but came with one aux and two phono inputs. Really, really would have preferred the other way around...
Now, that advice will cost ya. Ship me that Mac 1900 since you won't be needing it now. I'll be a nice guy and even pay shipping ;)