18 bits is a misnomer. SPDIF is up to 24 bit irrespective of whether speakers are 18bit or 96/24. With 18bit speakers, its the sample rate that is the limiting factor - 44/48 vs 44/48/88/96 - they will all accept 24 bit but probably ignore some bits.

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