One of the single ended el 84 amps, Almarro or Zen comes to mind. I use a Wavelength junior, but it is a bit pricey. Single ended el 84 sounded better than a number of other tubed and solid state amps that I tried with my ribbon tweeters. You could probably even use the el 84 amps from an old Roberts tubed tape reel to reel, which tend to sell at bargain rates.