How can I connect a pre-amp to my Onkyo TX N676 since it doesn't have a pre-out for amp

So were you saying that your Onkyo av receiver does not have pre-outs for you to connect to a dedicated stereo preamp with HT bypass? So I’m under the impression that you are going to get a separate stereo preamp and power amp to drive your speakers. And you will be using your stereo preamp for listening to stereo music and your Onkyo av receiver for HT?

Well.....if your Onkyo av receiver doesn’t have pre-outs, there’s nothing you can do. You can upgrade your AVR to the one that has pre-outs or perhaps get an AV preamp surround processor?

Thank you. The AV receiver accepts 4 ohm speakers.  I should be fine in my 15x20ft room 

No, your Onkyo receiver will not be able to drive a 4 ohm speaker load properly. You would need a dedicated power amp.