The original ESL was a notorious hard to drive loudspeaker. See the graph from Sheldon Stokes. A low impedance at around 10 Khz and of highly capacitive nature. The ESL 63 has a more friendly impedance curve, but the same behaviour at around 10 Khz. The Martin Logan approach, hybrid systems, will not help in this area. The problems are in the HF section, the LF section of a full range electrostatic is easy to drive. On the Sheldon Stokes site you can find a diagram of a dummy load electrostatic, very useful to test an amp without the risk of damaging your fragile ESL.
To improve the current output, and prevent high distorsion due to current clipping, the two channels of an amplifier can be connected in parallel (offcourse with some extra measures). This is useful when you use a 405 (2) to drive an ESL63. The improvement is als
o there when you use a 303 and ESL57 combination. In both cases the channel separation will be on CD level, which is not so if the individual amplifiers are used as a two channel combination. The 303 and 405 where built in a era when 25dB channel separation from your high end pick-up cartridge was top!