French Fries,
The Atma-Spheres and other output transformerless tube amps roughly approximate a "current source", unlike most amplifiers which approximate a "voltage source". In English, that means the Atma-Sphere amps deliver less than their rated power into a low impedance load, and more than their rated power into a high impedance load. This is just the opposite of the way most amplifiers (especially beefy solid state amps) behave. In addition, OTL amps typically have a high output impedance (low damping factor) which further makes a low impedance speaker less likely to be a good pairing.
Duke
The Atma-Spheres and other output transformerless tube amps roughly approximate a "current source", unlike most amplifiers which approximate a "voltage source". In English, that means the Atma-Sphere amps deliver less than their rated power into a low impedance load, and more than their rated power into a high impedance load. This is just the opposite of the way most amplifiers (especially beefy solid state amps) behave. In addition, OTL amps typically have a high output impedance (low damping factor) which further makes a low impedance speaker less likely to be a good pairing.
Duke