All other quality matters being equal, monoblocks are always worthwhile.
The Zu single FRD speakers have tremendous application elasticity for pure music uses. It's efficient and easy-to-drive and get good sound from a plethora of modest amplification options, beginning in the $$hundreds. But it has revelatory and tone density characteristics that can take good advantage of premium amplification many times the cost of Omen, Soul, Superfly, Druid or Essence. My Druids system has amps that are more expensive than what's in my Definition 2 system. Sure, somewhere there's a threshold where further expense in amps doesn't make sense for $2600/pr speakers like Superfly. But that threshold stretching credibility is much further out than most people would think, once you're committed to the holistic presentation of a single-FRD Zu system.
Essence is the problematic model in terms of pushing the amp/cost envelope, due to the mild loss of tonal intensity from the de-tuning of the FRD, and the discontinuity between the FRD and the chosen ribbon tweeter, but if your frame of reference is other speakers, rather than other Zu speakers, even that difficulty recedes. But the outer limit for amp cost to mate with Essence is nearer than it is for Superfly or Druid 4-08.
If you're not up for evolved 845 SET and their associated cost, a completely safe, tone-rich, beautiful choice for Zu speakers with 12-16ohms impedance is Quad II Classic (or rehabbed vintage) monoblocks. The Shuguang Treasure KT66z black bottle tube performs beautifully in them. The Quad II is the most SET-like push-pull tube amp I know, owing to its clever design simplicity and execution. And as hifi goes, cost is fairly modest. There are are others. I recently heard gorgeous sound from a Bob Hovland-modified Glow amp, its el84 5/5w easily trumping a number of megaprice amps, within the Glow's surprising dynamic range, on both Druid 4-08 & Definition 2. But $20,000 SET monoblocks don't sound like overkill either. Tremendous application range in amp options.
Phil
