I've received the Blu 2 and have tried the BluDave on my speaker rig at home (to Omega CAMs directly with HFC Ultimate Ref RCAs) and headphones at work (to DIYT2 and Sennheiser Orpheus HE90).
I must say that the improvements are immediately noticeable. It seems wrong to put it this way, but the BluDave lifts away the smudge around instruments and voices that was once there with just the Dave. Tonality is richer, with sonic cues presented in a palpable stable and solid sonic image. One way of putting it is that sonic cues which used to sound like they were floating in space with the Dave now sound properly like instruments playing with the musician sitting or standing firmly on the ground. Realism is the word.
To make sure I wasn't hearing things, I did an A-B comparison: I swapped the USB feeding the Blu 2 into the Dave, switched the Dave from BNC to USB mode, and the magic is gone.
I was most impressed with the effect on my headphone rig: I must say that whilst I had to use a tube amplifier with the HE90 headphones, which are very dated by today's standards. It shows that this old system could still scale higher with improvements on the digital front end. I have never heard anything as resolving yet. I listened to the BluDave all day at work (very distracted from it I must say) and brought the BluDave back home. The BluDave directly driving the speakers was less impressive, still very impressive, but I felt the resolution of my headphone setup could not be matched. Might be speaker placement or simply the speakers not up to it, I cannot be sure.
Wth both speakers and headphones another thing that was immediately apparent was the improvement in soundstage, depth and bass. Bass in particular was an area I felt the Dave alone lagged behind other top of the line DACs. I don't think so now. A typical criticism of electrostatic headphones like the HE90 is the lack of bass - I agree with that statement but, for the first time, I consider that it's bass was more than adequate in terms of quantity and viscerality when I was playing Jennifer Warnes' Way Down Deep. It's fair to say the BluDave has turned up the bass a notch or two.
The improvement in depth is very noticeable. I put on Yuja Wang's Rachmaninov (Rhapsody on a Theme of Paganini) and was able to hear for the first time that Yuja's piano was distinctly closer to the audience than the strings. With just the Dave, it seemed that she was much closer to the strings; with the BluDave the focus was much on her which is the point of the recording.
These are my findings so far. In short, the Blu 2 completes the Dave. It's worth it. There are things I could still improve - there's a little bit of glare (interestingly, mainly when I play back CDs). If anyone has extra ferrite cores or cable recommendations do let me know!