The Taboo Mk III and even more so the Taboo Mk IV with its UFO transformers are a great amplifier and very hard to beat. The SEP sound at its foundation is not fundamentally different than the SET sound of the Zen Amps. But there are differences in a very revealing system.
I had earlier Zen 2 watt amps, then listened to the SE34 amps, then the Torri Mk II and Mk III and then got a Taboo Mk III (still have it) as a headphone amp. When the Taboo Mk IV came out I bought one of those and yes it's a slightly better headphone amp, but it excels on the right speakers. (And I have not heard the Zus but am sure they would work well with the Mk IV). Then I bought 25th Anniversary SE84UFO3 Monoblock amps because of the great reviews from Steve and Joe et al, and I love these amps. Still I was very happy with the Taboo Mk IV driving the same speakers and can live happily with that combo (and live with the Mk IV in my second system now). In fact I think that the Taboo Mk IV is perhaps a bit more dynamic and focused. The Zen Monoblocks are perhaps more atmospheric and detailed. To some the differences would be subtle, others far more important, a lot depends on the system, and personal goals and tastes for musical playback.
You can't go wrong with either with efficient speakers. With the Taboo you get the added benefit of a world class headphone amplifier. And with the 25th Anniversary Zens you get a slightly higher powered microscope into the music.
Either way, to quote a song "the waiting is the hardest part."