I would think the Chord TT2 would be the best match as others have suggested. I base that on how it would work with your existing gear, using a tube amp and Harbeths, it would balance them out nicely. I think if I had neutral, analytical sounding speakers and amplifiers I would want an R2R DAC like Holo Audio to bring an organic richness, but with your kit the purity, accuracy and detail of a Chord DAC would maximize the sound of your other components, which have a character of their own that you like. And you can upgrade it with the m-scaler at a later date.
Edit: sorry you can ignore this comment, I typed it before I read your update about not using the Harbeths, I'm not familiar with the Dadealus so couldn't really help. But I think all these suggestions are great and your system should sound epic with a nice upgrade. Good luck in your search.