MkII had optical bias control on both S500MkII and S300MkII and were more reliable for it. I'm not sure what the "E" stood for
Look at the first response here,
Seems to be correct except for info regarding "e" series. The S300 series II was discontinued in '88, however never available new as an "e" version. The "e" series didn't come out till '90 with the S/350e being the 150w/ch amp. This series did have both pure class A and class AB amps. Any S300 that is claimed to be "e" series may have been upgraded at the factory during the 90's or upgraded during a recent refurbish by someone like Jon Soderberg.