Short answer: The typical “measured” max output power is 15-17W.
This is measured in a lab read: simulated, with a sine wave into a resistor load.
This is not reflective of what the real world loudness is, not the actual load.
Longer answer: It is not always advisable to measure valves and solid-state using the same “in lab” approach.
For example, Solid-State doubles the power each time impedance drops. With valves, power stays the same when impedance drops. They just are not the same.
Therefore, we have done our utmost to “equalise” the measurements by using an actual “music signal + speaker load” so the real world measured power is the same loudness (SPL level) for solid-state as it is for valves.
Most important of all this approach means a customer will find in reality, with the same music signal and on the same speaker, the iFi Stereo’s 25W+25W is the same loudness as a 25W+25W Solid-State amplifier. Not louder nor quieter.
The more lengthy explanation is to be found here:
https://ifi-audio.com/audio_blog/retro-stereo-50-there-are-watts-and-there-are-watts/
