I'm thinking of buying an Onkyo 609.If I did could I use it with my NAD stereo amplifier? I am thinking I could use the Onkyo to connect to the NAD for stereo playback using my Dali Lektor 2s but then revert back to the Onkyo for 5.1 playback
You can by treating the NAD as though it were a power amp and by utilising the Onkyo's front stereo pre outs, but this is tricky and fraught with danger of you accidentally overloading the stereo integrated amp.
It is probably better to employ the use of a dedicated amp switch/audio router: TC-7220 AUDIO ROUTER
This would allow you to have both amps connected to the speakers and let you switch between the two without having to rewire them.