I use Cerabase from finite element which yielded better results than the fairly nice stock cones.
I have not tried stillpoint for speakers. I have stillpoint racks under my amps and they are nice improvement.
I used to own the Opus Ceramique, then the Eidolon Diamond and finally the Isis.
Opus Cera is fairly easy to drive and so is the Isis. Your amps have no problems.
If the bass modes are properly addressed, there is no timing problems with all the Avalon models. If room modes are not addressed, the bass excitation can spoil the midrange + timing. I suspect the opus cera is designed for closer to wall placement, smaller rooms in asia or europe.
The ceramic bass drivers is limited in output. The bass alignment of the Indra is slightly suppressed as measured by stereophile. This would yield a even bass response with some boundary reinforcement. You are looking at a speaker with less bass output than the indra.
I would sway the opus cera is the most coherent because drivers are of similar material. It provides a taste of the Avalon imaging and resolution. Ulimately, It sounded too restrained in macro dynamics.
