I've heard, though only briefly, the Acapella. The tweeter is very nimble and nice sounding. The Hill I heard MANY years ago (around 1979). The Hill required a bottle on inert gas that is injected into the ion tweeter to prevent corrosion of the electrode that ionized the gas. You would get a wiff of ozone being discharged by the speaker.
A few years ago, the DIY magazine AudioXpress (at least I think it was this magazine) had an article on how to make your own plasma tweeter. Their design did not require injection of a gas around the electrode. The electrode had a quite limited life, but, it was cheap and fairly easy to build spares.
A few years ago, the DIY magazine AudioXpress (at least I think it was this magazine) had an article on how to make your own plasma tweeter. Their design did not require injection of a gas around the electrode. The electrode had a quite limited life, but, it was cheap and fairly easy to build spares.