I had a KRELL Duo 175XD amp (loved it) and the CODA 07x preamp (hated it). I came from the CODA CsiB and the KRELL K-300i (the latest Duo XD model). I currently have the CODA #16 (love it).
Separates are better in every case I tried. I tried many combos. With that great KRELL amp you have my fav was the Benchmark LA4 preamp. The Holo Serene KTE would also be good if you want to stay closer to the 07x sound. However, the 07x and KRELL is not my favorite combo at all. The 07x is noisier than the 2 preamps I mentioned (by a lot) and warmth on top of warmth is too much heat for me.
Try the LA4 on a 30-day home trial or get one cheap on USAM.
I also have a great streamer in the Lumin X1 (sold it) and the Sonore OpticalRendu (kept it) when I had the KRELL Duo 175XD. I only do Fibre Optic streaming. The RJ45 streaming on the KRELL K-300i was very convenient but not as good as Fibre.