High-Efficiency Single Driver Speaker System

For those of you who have gone to the 'Dark-side'---a high-efficiency single driver system and never looked back, I'd like to get your feedback on two specific things: 1)What type of music do you mostly listen to, and 2)What are the most appealing characteristics that you find lacking in more traditional lower efficient, high power amplifier-based system. I currently have a custom EL34 JWN amp rated around 35-45 watts and I'm considering going this route after having been in the more traditional camp for over twenty years. Thanks in advance for your feedback.
I don't think there is a "single" speaker with that capability. But there are certainly coaxial two way speakers out there. GPA, Great Plains Audio, has upgraded versions of the Altec 604. Tannoy is still in business. Probably others too.

I do like high efficiency speakers. Mine are not coaxial.

I do have one low efficiency pair of speakers.