@Nostromo
a Spanish Amiga user has a BenQ G922HDL (LED backlight monitor) connected directly to his A4000 without scandoubler! He has written that it doesn't handle interlaced modes well and that he has not got 4:3 working (I guess he has not found the option to switch to 4:3 like you did because both monitors seem to have similar controls)
Perhaps BenQ monitors are the future monitors of Amiga users? It's not my favourite brand but finding one monitor that accepts 50Hz is difficult. If it accepts 15Khz is a dream come true!