I am not good at putting words to what I hear, but let me give it a try. This is on my SR-007 with blu-tak mod.
The Carbon is more crisp, but more 'in my head' in terms of sound stage. The Blue Hawaii has more body and a noticeably wider sound stage. While the Carbon appears to highlight higher frequencies, I find that cymbals and the metallic 'sizzle' sound of a snare sound better on the BH than the Carbon. Backward from what I expected. The reason I tend to really like the BH is that it excels at warm and full, but does not sacrifice high frequencies and fast transients in any way. Its ability to do both so well is what struck me the most as I started to get used to it.
I can make the BH sounds more like the Carbon and vice-versa by using frequency equalization in foobar player. So it appears much of the difference I hear is related to which frequencies each amp peaks on. It is also interesting that I was expecting the Blue Hawaii to sound more like the SRX-Plus since they are both tube amps. But I find the SRX is just as different from the BH as the BH is from the Carbon. 3 very different amps.
With flat equalization, I tend to like the BH best right now. I listened to the BH for 1 hour, then the Carbon for 1 hr. Not scientific, but I just enjoyed the BH better. As JimL has said, it is always the new girl who gets the most attention, and perhaps that is what is going on here. After building the Carbon, I felt it was better than the SRX, but after a few weeks I found myself using both pretty much equally.
I am very happy with these 3 amps and am indebted to this community for all the help in choosing and building them. I am also glad I have all 3 because they are all different and great in their own way.
EDIT: FYI, my source is a Gungnir Multibit ('Gumby') using balanced out into both the Carbon and the BH.
Now on to more listening.....