Can't confirm on D6k's, but on D5k's, pretty sure v4.x was the last EEPROM version, everything after that was flash upgradable - presumably that was the 18bit to 96/24 transition..

Meridian owner since 1992
Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).