I've heard Viva and Mastersound big SET on Zu speakers other than Essence. Have listened to Essence on Audion 845 monoblocks and Audion Golden Dream PSET monoblocks. Both of those amps are 25w. The Viva is euphonically old school. Not as illuminating as you should get at their price. The Mastersound 845 I've heard was more credible for being musically convincing. Audion handily elbows both aside.
A PX25 SET amp will generally sound quite clear but will run out of steam in a larger room, especially with the 4db less efficient Essence. 20w of SET is a clear dynamic improvement, all other things being equal. But just as my 300B PSET amps are not quite as dynamic as my 845s, despite the same power rating, remember it's the "drive" more than the steady state power that matters. Put a first class 845 SET on any Zu speaker and you're done. But that's not cheap, "first class" being the determinant. I do find the Quad II to be an excellent p-p step back from SET, for the 12+ ohms Zu speakers, with the more complex Quad II-80 and II-40 being highly credible at more power. Use the Shuguang Treasure power tubes. In this pool of amps there's bliss to be found.
Phil
A PX25 SET amp will generally sound quite clear but will run out of steam in a larger room, especially with the 4db less efficient Essence. 20w of SET is a clear dynamic improvement, all other things being equal. But just as my 300B PSET amps are not quite as dynamic as my 845s, despite the same power rating, remember it's the "drive" more than the steady state power that matters. Put a first class 845 SET on any Zu speaker and you're done. But that's not cheap, "first class" being the determinant. I do find the Quad II to be an excellent p-p step back from SET, for the 12+ ohms Zu speakers, with the more complex Quad II-80 and II-40 being highly credible at more power. Use the Shuguang Treasure power tubes. In this pool of amps there's bliss to be found.
Phil