Darkglass FAQ says all post 2015 pedals except the compressor and the latest Ultras with digital gubbins can run at 18V. My B3K is older than that, so I got lucky when I tried 18V by accident. Others have been not so lucky, so try at your own risk!
The M900 preamp runs at 15V. I believe this is why so many people say they prefer it to the tone they got from the pedals. Ran my B3K at 18V and boom, got them to sound identical. Less fizz, more dynamic range and touch sensitivity to the overdrive.
I mentioned to Doug on the TB thread about adding a 9V/18V switch to the plugin and he said they'd look into it!