From the H9 User Manual, p47:
The unit is a complex piece of equipment that does not react well to being dropped, bounced, crushed, soaked or exposed to excessively high temperatures, voltages, electrostatic or electromagnetic fields.
Notice voltages.
I think I would err on the side of caution if it were me.
Here is a link to the the manual. Use the “Find” function on your browser or PDF reader to search for the word volt.
There are 5 instances and none of them mention hooking it up to an 18 volt power supply.
9v is the highest voltage mentioned in the manual.
https://s3.amazonaws.com/com.eventide.downloads/Product+Manuals/H9-UserGuide.pdf
