Anyone listen to Zu Audio's Definition Mk3?

So Gopher, and Phil, what's going on with such a dramatic change going from so-so sound to ground breaking with a change of tubes?
The main reason I ask is that my Def4s are finally being installed in a couple of weeks, and my pre (Hovland HP-200) is tube. I'm running them with 1950s NOS Telefunkens.
I'd hate to have to investigate new amp combinations at this time, and I'm not the best person to tube roll.
Btw, LOVE these tubes, they sure are fast, but not too clinical, in my current system.

Spirit,

The balance and presentation of the Def 2 and Def 4 drivers are night and day. The Def 2 is darker, less resolved (and less exposing) richer (I think so) and far more laid back. The Def 4 on the other hand is open, extremely revealing, super transparant and a lot more lively.

The stock 211 tubes I had were harsh, unsophisticated and had a good bit of glare to them. Changing them for the 211Bs the horrible glare (which the Def 4 drivers ruthlessly laid bare) and adding dimensionality, beauty, body and tone made a HUGE difference.

Phil will probably speak for himself, but he told me he had a similar experience with 845A vs. 845B tubes in going from Def 1.5s to 2s to 4s.

I have some GE 211s which are supposed to arrive tomorrow, will be interesting comparing them, though I'll likely have to change my driver tubes.

Thanks Gopher. I found the stock tubes in my Hovland pre were always a bit plain and undynamic with the Def2s. This was remedied by going the NOS Telefunken route, opening up transparency, sparkle, speed and dynamics, but not sacrificing analog warmth.
It looks like they'll be an ideal match for the Def4s.
Additionally, my new direct rim drive tt/air bearing linear arm is showing my cd player a clean pair of heels. From your comment, this gap may be widened still further.

Received my Def 4s yesterday. I seriously considered upgrading my old 2s to 3s, but this thread pushed me to the 4s. As previously stated, this is a totally different speaker from the 2s. In my room there are huge improvements in all areas. The new super tweeter gets a big thumbs up from me. That along with the increased resolution from the FRDs gives you so many more spacial cues that the whole musical experience is dramatically better than the 2s. The woofer is way better also. On the down side, these things are HEAVY! I almost had a hernia getting them out of the box and set up. Of course if I would have waited for my wife to get home from work and help it would have been a lot easier, but there was no way they were going to sit in those boxes all day with me looking at them wondering how they sounded. As an FYI my 9 wpc Yamamoto 300B amp drives these with absolutely no difficulty. More to follow.

If someone wants a great Zu amp, check out the Mastersound Due Venti on the 'gon right now. Gorgeous sound (and looks) with Siemens EL34s in particular. No better cheap amp I've heard!