Thanks for your suggestions. I should have mentioned that it takes 4 RCA plugs to hook up this sub. L/R Input and L/R Output (High Pass).
But that is the whole point of the Sub output on your new Denon receiver. it does not need 4 RCA plugs.
Your old system you were feeding all the sound (left & right) out the tape out port to the sub amp where you did your high pass filtering to pull out just the low sound that the sub would play, and fed back through the tape input your high frequency sound that you would play through your normal speakers.
But the Denon doesn't need any of that. it has all the logic built inside itself that old receivers did not have. it can internally do all the high pass/low pass filtering along with digital source material that has encoded LFR (low frequency range) data. So inside the setup of your Denon, you specify at what crossover point you want the sound sent to your sub. You don't even need to run a left and right as the Denon inernally looks at all the channels for you.
You really should (RTFM) on your Denon as it should have covered all of that in the section about the sub.
You old DIY sub setup amp might be more happy with a left and right input, so you can use a Y-connector or as the Denon has two sub outputs use them both.
You don't need anything running back to the Denon from your sub as there is no high pass filtering going on. simply ignore that cable as it is now redundant.
