I have a pair of Zu's now-discontinued Rune model, which I had upgraded in April to Druid Mk4 specs with the latest drivers, internal wiring and back plate. The cabinet design is the same for both models, but differed in the method of construction.
I have been very pleased with them, great coherence and balance. They are fast, dynamic and intimate. I'm driving the Druids to great effect with a 2 Wpc tube amp.
They are rated down to 40Hz, which is plenty for me. Even with just 2 Watts of tube amplification, the bass is not flabby or out of control, in my system and room, which is admittedly on the small side. Previously, I was using a 125 Wpc solid-state amp in a larger room with very good results also, not better or worse, just different.
Apparently, the bass response can be lowered slightly by using shorter spikes so that the base is 3/4" from the floor. Currently, mine are about 1 1/4" from the floor and I don't feel the need to lower them, but I may try it eventually.