I own an EAD Theatremaster Ovation 8 pre/pro that uses this very same DB-25 connector for multi-channel analogue pass-through. When I first had my pre/pro upgraded to facilitate multi-channel analogue pass-through ,EAD included a DB-25 to eight RCA pass-through cable. I found the quality of this cable to be wanting however. Right about this time Alpha Digital took over EAD. One of the other companies that Alpha owns is Tara Labs, a cable manufacturer. Alpha had Tara Labs create a pass-through cable for the EAD processors. While this cable, which EAD supplied to me for a nominal fee, was an improvement over the previous throw in cable,I was still not very happy with the sound quality. Tara used one of its entry level interconnect cables for this pass-through. Just by chance,when I was at a local dealer one day, I spotted a box behind the counter that contained a six channel pass-through cable with a DB-25 connector made by Monster. Although I have never been a big fan of Monster Cable I could tell upon inspection that this was a much better built cable than either of the ones EAD had provided. The dealer was blowing it out, due probably to the rarity of need for this type of connector. Excitedly I brought the cable home and hooked it up to my pre/pro. Imagine my surprise and dismay when no sound emanated from my loudspeakers. I checked all of the connections, and everything was correct. Hmmm,.It then struck me to check the owners manual of my EAD processor. Sure enough there was a page which diagrammed the pin assignment for the DB-25 connector. I contacted the dealer and told them what had happened. They agreed to re-solder the cable in the proper configuration for me. This cost a hundred dollars, so much for my bargain. Finally after another mistake where the wrong sub pin was connected , I got the cable from the dealer and installed it in my system. Now this was more like it. Much better sound overall, and to think from a Monster Cable.
Now, finally to my point. Without knowing the exact channel designation for the pins of the DB-25 connector it will be extremely difficult, maybe even impossible, to re-wire the cable properly if it needs to be. The Monster cable I bought was wired in accordance to THX specifications. If your Onkyo is THX certified, it is likely that the pin designation will be the same as the Monster Cable or any other similar type pass-through cable that has THX certification. If not, without knowing the pin designations, you may be out of luck. I find it hard to believe that no one at Onkyo/Integra can find out this information for you. That's inconceivable.
These monster cables, the model I have, and one model above that one that they made, are no longer available. I know that Wireworld used to custom make this type of cable as well but I'm not sure if they still do or not. Either way without knowing the pin designations I don't see how you could possibly even get an appropriate cable made. There must be someone somewhere that can furnish that info to you, or better yet someone who owned that same unit and is willing to sell you their cable. Are you sure there is no diagram of the pin designation in the Onkyos manual?
Now, finally to my point. Without knowing the exact channel designation for the pins of the DB-25 connector it will be extremely difficult, maybe even impossible, to re-wire the cable properly if it needs to be. The Monster cable I bought was wired in accordance to THX specifications. If your Onkyo is THX certified, it is likely that the pin designation will be the same as the Monster Cable or any other similar type pass-through cable that has THX certification. If not, without knowing the pin designations, you may be out of luck. I find it hard to believe that no one at Onkyo/Integra can find out this information for you. That's inconceivable.
These monster cables, the model I have, and one model above that one that they made, are no longer available. I know that Wireworld used to custom make this type of cable as well but I'm not sure if they still do or not. Either way without knowing the pin designations I don't see how you could possibly even get an appropriate cable made. There must be someone somewhere that can furnish that info to you, or better yet someone who owned that same unit and is willing to sell you their cable. Are you sure there is no diagram of the pin designation in the Onkyos manual?