I've heard Zu's with valve amplification (think it was Purite North/EoS with EL84s at Scalford) and they sounded fantastic. The combo had all the good valve stuff, plus nice and pacey. Also, Audio Counsel in Manchester sell Zu speakers with Audion EL34 amps, if that is any indication. Your Art Audio should be perfect for them, I would have thought. Certainly a much better match than the ATCs.