Hi Murray,
I have not tried the D600.Is actually the only piece of TAD reference series that I haven't tried in my house. Can't say how it compares to the Grandioso. I have my opinion of course but it is not based on anything objective so worth very little. Interestingly, the TAD head for Europe was in my house a few weeks back and he told me the best selling of all the hardware of TAD was the D600, and it was quite popular specially in Japan. On paper the multi box solutions from Esoteric Grandioso or Vivaldi seem better, but paper and specifications many times means nothing to how it sounds, son can't give an opinion.
Regarding Siltech, I have tried and used several ranges within the Royal Signature Series. From a Queen Interconnect, to an Emperor II speaker cables, Double Crown Interconnects and TC speaker cables and interconnects. Also DC power cables and DC Octopus power bar. They all sound great with Ref series. My opinion is that sweet spot is probably Emperor (price/performance). Of course DC and TC are better, and you can easily hear it in this system, TC being a revelation, but with the Emperor, if you don't listen to anything above you will never imagine you are missing anything. Queen is also good but only for preamp to amp connection. From DAC to preamp, where signal is more delicate it is better to be Empress or above.
It's hard to say but if I had to recommend a combination not to max performance (which would be al TC) but max cost/benefit I would say from DAC to preamp highest you can afford (TC or DC), from preamp to amp (queen or Empress, the later better if possible) and speaker Emperor II. That will be excellent and again you would not be able to see what it is that you are missing if you don't hear higher combinations. (of course you are; but you are anyway regardless of how high you go...).