The new version with the UFO transformers may change that since they have 8 and 4 ohms taps???

The older versions had a lower imp. transformer stock with the option of a different trans for higher imp. speakers.
I have the Super Zen Select with the UFO mod. I was using the Zen Select with the stock transformer for a year and half before upgrading to the UFO transformers. I want to clear up some misinformation regarding the little Zen's speaker ability.
All of the previous little Zen Triodes with the stock transformer had a 6 ohm tap ( not 4 ohm as previously discussed here) that could handle speaker impedances all the way down to 1 ohm with no issues whatsoever. The optional ( or EX mod as was called) transformer was designed for higher speaker impedances 6 - 16 ohms. This optional transformer also had a 6 ohm tap. The reason for the differences are the stock transformer had a 9800 ohm primary and the optional ( or EX ) transformer had a 3300 ohm primary. Both had a 6 ohm secondary.
The UFO transformer, in addition to being much better made in terms of parts and precision, combines both the stock and optional transformers into one through the addition of a switch. The UFO has a 6600 ohm primary set against a 4 and 8 ohm secondary that can be selected through a switch near the speaker binding post.
This means that in addition to being much more refined (better sounding), the UFO can handle speakers 2 - 16 ohms with no issues. 4 and 8 ohm speakers are ideal because you will get the full 2.3 watts of power from the amp. 16 ohm speakers should also work well but will see 10-15% decrease in power.
So getting back to the topic of the thread, using the 4ohm tap of the UFO there will be no issues at all and in fact Steve intentionally designed his transformers for this unique ability of a SET amp ( of being able to handle dips all the way down to 1 ohm).