Retrovibe Stinger - 9v or 18v?

[Norris]

I fitted a Retrovibe Stinger preamp to my Sterling Sub5 and it's transformed it from an indistinct mush to a gorgeous sound. However on my latest gig towards the end of the night it felt like it was running out of steam. The Duracell battery has been in for a few months, so it might slightly past its best. Alternatively it might be running out of "head room".

 

Has anyone got any experience of running this preamp with 9v vs 18v? The bass has a single battery compartment. I'm fine fitting a dual compartment, but would it be worth it? When the battery gets a bit low does it suddenly die or gradually fade away?