Another option is MS600 to up the ante on audio performance.

As for core, everything that I have seen with Sooloos points to store+core in same unit being optimal. Options for this are MC200 (basically your C15 without control), MD600 (TwinStore replacement and probably best option), MC600 (modern multizone 'Ensemble').

As you say, your C15 could easily be replaced with a C10 functionally, but would you get any money out of the downgrade? Probably not enough for it to be worth it especially as the C15 is probably a better C10 than the C10 is.

As for analogue pass through. Most modern Meridian equipment is inherently all digital so any preamp type of device (861/818/808.3 etc.) will to ADC to get signal into the digital domain as soon as possible. Most people, having heard the result are not bothered by this.

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