The already-mentioned Music Reference is unique in that the company offers an ESL loudspeaker/dedicated OTL tube amp combination. Not only is the amp output transformer-free (hence the OTL designation), the ESL is stepup transformer-free; the amps' output tubes are connected directly to the ESL drivers! Designer Roger Modjeski created a similar design when he worked for Harold Beveridge.
Not yet mentioned is Sanders Sound Systems, who make a fantastic ESL loudspeaker and a solid state power amp designed specifically for ESL's. A second version of the amp is optimized for magnetic-planar loudspeakers (Magnepan, Eminent Technology).