Upgrade or Downgrade Changing Bel Canto 3.7 For New Denafrips Pontus 15th?

I believe you have a fundamental problem with what you are trying to do.  The Onkyo C5 CD/SACD does not have an IS2 output unless you have had it modified? The S/PDIF outputs (optical/Toslink, and Coax) do not output a native DSD signal. 

Am I missing something?

@ucrazykid 

You are right in what you say but my SACD playback limitations are confusing.
The Onkyo is only present in my system as it will play SACDs as a 'native' SACD player outputting an already decrypted digital signal to the Bel Canto through toslink. My main CD player however is the PS Audio Perfectwave but it is just a CD/SACD transport, not a player.
My wish is to use the PS Audio Perfectwave for all SACD playback too, but the SACD transport signal is only playable through an I2S connection (Sony Copy Protection limitation). The Bel Canto 3.7 doesn't have an I2S input, hence potential move to the Pontus 15th. that has this input (and whose pinouts can be tuned to connect to the PS Audio unit) as there is no standard pin configuration for I2S connections!
The new setup would then be PS Audio Perfectwave playing both CD and SACD connected via I2S connection to the Pontus 15th. The Pontus 15th then connected to my Kratos amplifier through balanced XLR. So you are not wrong with your comments about the Onkyo.

Ah! I see. So they Onkyo won’t be needed once you have the PS feeding a DAC from its I2S output.  Got it. 

Why not a PS Audio Direct stream Mk I. They're selling reconditioned ones for $1600. 

@dickieboy 

That should guarantee integration but I have tried the PS Audio and didn't love the sound and for just $300 more I can get a new Pontus 15th which is really an improved Venus 12th that's almost as good as end game Terminator. But a sensible idea to go PS Audio, love their BHK range.