If the source is hooked to the NAD, you can use the tape monitor out to the analog input on the Sony. Hooking things up this way will mean that you will control the volume of the speakers hooked up to the NAD via the NAD, and the speakers hooked up to the Sony via the Sony. If you want one volume control controlling all of the speakers, you would hook up the preamp output of the NAD to the analog input on the Sony, and then the NAD volume control will affect both, and to balance between them, you would use the Sony volume control.
If the source is hooked up to the Sony, you will need to have an adapter for your headphone jack on the Sony (since that is the only audio output on that system other than the speaker outputs) to go to any analog input on the NAD, making sure that you do not have the Sony at too high of a volume that you overdrive the NAD input. In this case, the Sony volume control would affect the volume of the speakers from both, and you would use the NAD volume control to adjust the balance between the Sony and the NAD. Please note, this is assuming that plugging into the headphone jack of the Sony does not automatically shut off its speakers. If it does, then this will not work. The only other option would be to use the speaker outputs, and for that, you will need something that will step down the power so that you do not destroy the NAD. Do NOT attempt this without a special device, or you will damage things.
I do not particularly recommend that you do any of this, but this is how to do it with your particular gear. Going from the NAD to the Sony [in the manner described in my first paragraph], though, would be safe to do.
