I use a 300b SET with my Zu Def4s, and the sound fills my large space without any difficulty. It depends on the tube, the design of the SET, the output power, and many other factors including the room and (in my case) a very long run of speaker cable. If you are interested, you should call Zu to discuss. As I have learned, almost every SET is different and should not be lumped together. I had Def2s and tried 45 and 2a3 tubes before settling on 300b. Others swear by 845s. Other speaker considerations include the soon to be released Zu Druid 5, Audio Note, and Living Voice.
