Single driver speakers. Are they worth considering ?

According to their published specifications, Cube Audio’s 10" driver is flat to 30hz: https://www.cubeaudio.eu/cube-audio-f10-neo (but does drop off at the top end above 10k)

Cal,
Agree .
The PHY-HP is a legendary paper cone driver created by the late Frenchman Bernard Salabert. The 12" version in particular is held in high esteem for superb sound8 quality. The Cube Audio Nenuphar is a very different design and approach which by all accounts extends deeper and goes higher than the classic PHY.
Charles

I can't remember the name of speaker brand that uses 10" PHY driver and optional super tweeter. Or maybe there are a few brands.

Hi. I have done some short term  listening to the Cube Audio Bliss, Magus and Nenuphar at a local retailer in my area. I have never been attracted to single driver technologies as my past experience with a few designs indicated a very thin, one dimensional sound that I simply was not interested in. Unfortunately, I only had approx. 30 minutes of time to spend, but I will say this, I will come back for another  audition when time permits. There was something very different about this brands general sonic delivery. 

@inna,
Ocellia and Tonian Labs are two brands that have used various size  PHY drivers. There were probably a few more. 
Charles