meraklya,
I hope you get the Zsage before the holiday season! I look forward to your impressions of the newer Yakin buffer and your eventual comparison to the Zstage. Not having a Zbox, I can't directly answer your question about the similarity to the Zstage, but I hope this discussion has illustrated that the bias and gain of the Zstage do contribute to fine tuning your sound, including warmth, but also a whole lot of other aspects of the sound as well. If you feel like you need better clarity though, you could call Steve Deckart. He can describe quite articulately the differences for you.
Fireblade,
As I said, I personally am not sure I can overdue even order harmonics. Maybe, but I have not experienced it yet with my setup. This and other tube attributes that can smooth, richen, define, and give spaciousness...these are all things I like.
With my Torii and Zstage/ZDAC (my main setup for a long time), or with my modded Oppo, or Rega Apollo, I have not discerned this to be an issue for me. Nor have I found the Zstage to improve one source more than another. Each source starts differently, and the Zstage works with that particular complexity. It has improved all the sources I have used it with, but I like ZDAC over the others without the Zstage, and I like it better comparing all three with the Zstage also.
"Signature impact aside", once in use, I can say I have found the Zstage "unnoticed or marginally evident" in the sense of problems with the sound. I have found improvements evident.
I suspect it is mostly the tube and caps that causes the Zstage to "Flush out the detail ... improve imaging" with the the bias and gain adjusting this base sound, and the rest being as transparent/musical a signal path as possible. Since the tube is so powerful in the equation, the individual tube makes a mark.
I can't say at this point if the effect on the Tranquility will be more important but I doubt it. I really do love the nuance of the Tranquility, the nuance being made up of very subtle and compelling micro and macro detail, micro dynamics, pleasing balance, and so on. Being a NOS DAC, no doubt subtle harmonics play a roll in this nuance, and whether I find the Zstage to improve the sound remains to be seen, but considering all I love about the Zstage sound and functions, I would be surprised if I did not like it once I dial it in to my tastes and system. Until I can experiment though, I am just not in a position to say.
I wonder Fireblade...you have given a lot of thought and research to the Zstage, liking the concept quite a lot from the onset, and it seems to me that it is possible that you have made your decision conceptually. I suspect that after you get your Decware experience under your skin, the answers will be clearer!