Are you talking theoretical or practical? Have you actually heard the Alexx driven by an SET in the 30 watt to 45 watt range?
Both theoretical and practical. For theory you have the Stereophile measurements. As I listened to the Alexx sometime before the review, we tried it with the Audio Research REF 150 (4 KT120's push pul, 150W) that I owned at that time and it could not drive them properly, although it could easily drive the XLF's. We moved to Momentum's to listen to them.
Believe me, the Alexx needs real power. As said in Stereophile measurements "The impedance is specified as 4 ohms nominal, with a minimum magnitude of 1.5 ohms at 2.85kHz." This kills any 30W SET - electronics can not make miracles. Read more at https://www.stereophile.com/content...-loudspeaker-measurements#CAwkhjum4o31HBpd.99