Just recently, I think I have determined that silver-plated conductors for digital cable is actually the better way to go. (at the same time, I still think that silver should be avoided for interconnects and power cable). This was from comparing a Beldon "certified 4K’ hdmi cable to a Neotech silver-plated hdmi cable. The Beldon was definitely smearing the sound, causing problems in the digital signal. The Beldon also rolled off the highs and had weak bass.
So, your experience with the new D-750 (which is a silver-plated solid-core conductor) definitely helps my theory.
As far as your situation, I do think that putting a D-750 on the pre-DIP stage would help. In my experience "garbage-in-garbage-out" is definitely true. I also have seen where power cable on a digital transport can affect how good the sound quality is.
How long are each of your cables? I usually make sure it’s 6 foot to avoid reflections (even if it’s silver).
The Wire World Starlight 7 might be an interesting cable to compare. The DH Labs uses a single solid-core conductor, which has been known to work well because of the 75ohm impedance dependency. However, when looking at digital signals, it’s a square wave signal so it’s important that the cable is able to charge and discharge the voltage as fast as possible. The multiple solid-core conductors of the Wire World cable might do that better than a larger single solid conductor like the DH Labs. In the end, I don’t know -- you’d have to listen to each. he. Both use silver coated conductors.