I purchased a new S5.5 last year and the Coda dealer told me he thought it is a step up from the #8 if you have efficient/easy-drive speakers, which seems to fit what is said in the Stereo Times review https://www.stereotimes.com/post/coda-technologies-class-a-s5-5-amplifier-by-terry-london/ when comparing it to the #16.
I've not heard the #8 or #16 so can't comment, but my last three previous amps were the Pass X250.8, XA30.8, and Atma-Sphere Gan Monos, and with my very easy to drive speakers I would say I prefer the S5.5 overall. It is very detailed and dynamic without being harsh or cold.