Home theater bypass

Got it.  If you unhook the RCAs from the tuner input and just run the Panasonic in stereo through the aux input does the CJ still trip its protection circuit?  If not, unhook that from the CJ and hook the preouts from the Onkyo to the aux input and see if it still trips.

Ok, so the Tuner input is your HT input.  Do you have a dedicated 2-channel source or is the DVD player used for both stereo and movies?

@audiomaze -- what input on the CJ are the audio outs from the DVD player connected to?

Please tell us exactly how you have this whole thing hooked up.  Something's wrong somewhere. 

You connect the CDP to any line level (non-phono, let's say the "tuner") input on the CJ and the L/R preouts from the Onkyo to another line level (let's say the "ancillary") input also on the CJ.  When you want to listen to 2-channel you choose the "Tuner" input on the CJ and when you want to do surround through the Onkyo you switch the CJ input to "Ancillary" and adjust the volume on the CJ to the reference level that keeps the L/R speakers balanced with the center/surround speakers.  That's basically it.  Just to be clear, only the preouts from the CJ go to the Mac 275. 

It depends.  When you're doing surround the AVR controls the volume, but for 2-channel not only does the stereo pre control the volume, the AVR is COMPLETELY out of the playback chain.  This is the real benefit of this setup.  Instead of swapping cables to switch between stereo and multichannel you just change the stereo pre to the input that takes the L/R preouts from the AVR and set the CJ's volume control to the proper reference volume level and you're done. 

BTW, while the method by @imhififan is maybe the more accurate way to go, another and perhaps easier method from a practical standpoint is to use the 12:00 position on your CJ as the reference point for the input from the AVR and then use the AVR to set the correct volume levels for each channel.  This method doesn't find unity gain for the CJ, but it avoids having to mark the proper volume level on the faceplate of your stereo preamp.  Hope this helps, and enjoy your new setup!  I did this for years before I got a dedicated listening room and it was awesome. 

What model CJ preamp do you have?  I thought they've been including HT bypass on their preamps for years.  Let's just start there.