Zanon: Soul gets *more* bass extension from the smaller cabinet than Druid, because of the Griewe model working inside a tapered cabinet. They've also eliminated, in a different way from Essence, the Druid's ultra-sensitivity to the floor-to-base gap when dialing in bass balance. You just set Soul down and roll some signal into them.
As for flea amplification on Zu: It does work. A 2w 45 amp will sound beautiful and be as or more dynamically suitable as any number of other HE Fostex or Lowther-based designs. But 2w are 2w. So the second watt gets you to 104db/w/m -- that doesn't mean you're getting ease and dynamic integrity. A solid 20-25w 845 amp will drive them fine in nearly any practical application, but not all 25w amps have the same apparent drive. A 25w PSET 300B amp will be dynamically less compelling but possibly tonally richer still. In a push-pull amp you might not be happy with less than 40w, but a pair os 15w Quad Classic II sound sublime and alive. Generally in solid state, you'll want more oomph unless the amp has an oversize power supply and lots of headroom for transient clarity. Yet despite the efficiency, a pair of McIntosh MC501s would not be out of place with any Zu speaker. It just depends on the actual behavior relationship between a given power amp and the Zu FRD. Just keep in mind in this case, the Soul's impedance is higher than prior Zu speakers. Up from the Druid's 12 ohms to 16.
Phil