I'm sorry but I can't be of help on the IR input on the receiver. My TX SR573 does not have this input, and if it did I am sure I would have messed around with it by now.
I've had a look at the manual for your 604 and it appears the jack goes straight in the back, bypassing the box and emitters on the hot link i recommended. With the Jack in the back of the 604 from the IR Receiver, the Onkyo RI function then takes over to control your Onkyo components. The only problem with this is if you have other equipment tucked away, for example, a Cable box. These would still require what I call an emitter from the hot link to make it function. Therefore, the cable plugged into the back of the 604 would have to be put into the hot link box and all emitters hooked up.
I hope that helps, rather than hinders. Let me know how it works out, I am curious.