When I had Zu Def 2s, I tried 45 (Yamamoto), 2a3 (DeJa Vu Audio), and eventually 300b (Ancient Audio) SETs. In part because I have very long speaker cable runs - probably 50' or so from one side of the room, under the floor through the cellar, and up the other side of the room - and because I have a huge listening space, I found that the 45 and 2a3 SET amps lacked necessary oomph. A 8 watt 300b SET was about the lower limit of what I considered acceptable.
I now have Zu Def 4s and find the combination with Ancient Audio Lektor Prime and a custom 300b SET (an updated version of the Integra that Jarek no longer makes) with Takatsuki TA-300b tubes to be great. (I have not gone back to try lower power SETs with the Def 4s, however.) If you go the 300b route, do not underestimate the impact of these Japanese 300b tubes; I tried Shuguang Black Treasures that came with the amp and Sophia Royal Princes, and the difference is significant.
As far as Ancient Audio is concerned, I like their gear; but it can be a bit quirky. Jarek built a novel (for him, at least) protective circuit into my 300b SET. When I first tried the Sophia and then Takatsuki 300b (rated at 60-65 mA), the circuit tripped within 60-90 minutes; and the amplifier shut down almost immediately. Finding a pair of 300b with a lower mA rating prolonged the listening time before it shut down, but not by much. I had Deja Vu Audio look at it; they replace a few small parts and changed the bias; and I now can get about 4-5 hours of listening before the circuit is activated and another 2-3 hours before the amp shuts down. This circuit may have seemed to be a good idea to Jarek, but it is frustrating. I have not listened to either his flagship 300b PSET or his 6c33 based amps. However, I must confess that despite all of the other issues, I enjoy the sound as well as the fact that they are "unique" - especially in North America. Conversely, dealing with problems between Washington, DC and Krakow can be frustrating. From your list, I assume that you are in Europe. Jarek welcomes contact with potential customers (although he is sometimes slow to answer email) and is very hospitable; I bought the Lektor Prime CDP after listening to it in his home in Krakow.
I heard the Yamamoto 45 SET a few weekends ago with the Def 4s, but it was in a small room. Two other amplifier manufacturers that you might consider are Sophia Electric and Border Patrol. Phil (Cobra 213) thinks that 845 SET amps - specifically either the Audion or the Sophia Electric - mate extremely well with the Def 4s; I have not heard either.
