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Spatial and Zu are great speakers but can also take a lot of power and play loud so I'm not sure they are the ideal match for low powered SET amps. Omega are also very good, have no crossover, and are designed around low powered amps. SET amps are also excellent when used in a multi-amped horn system.
For speakers like Zu, if you want to stay SET I'd look into larger models using high voltage tubes like 211, 845, 805 and GM70. Then you'll have the power to drive them more fully.