I've owned several Quads (though not the 989's) and both own and sell Sound Lab full-range electrostats. In amplifiers, I sort of specialize in models that do a good job on electrostats, and have several Quad owner among my customers. I have customers who have been very happy driving their Quads with amps from Atma-Sphere, Wolcott, InnerSound, and Parasound (the JC-1's). I sell these amps, but by no means are these the only options out there. For instance, the Thors mentioned by Thorman would also work quite well - for a while I drove a pair of original Quad ESL's (the "57's") with a pair of TP30s, which was about as big an amp as those speakers could handle.
Since you are open to tubes as an option, my suggestion would be to go with a lively-sounding tube amp (either OTL or with a high quality, wide bandwidth output transformer like the Wolcotts or Thors).