tip: don't get too hung up on the 18v or 9v. What basically happened was that I could hit the "red mark" i e clip the digital input on my soundcard interface, and input level, every knob set the same as with at 9v. So if you back off the knob so it doesn't hit red, you only get a larger headroom, and the noise floor gets a little less. but nothing actually happened to the dynamics, since that is what the compressor does, to tame or "pop" the dynamics. It works OK alright on 9v. I will not mind doing that at all if running into any amp live, maybe distorting the input a tad... this clipping ONLY occured when running an active bass at 18v battery too and on top of that only when BANGING THE LOW B, furiously. I could not make it clip at 18v with my fingers only.
So the differences with 18v vs 9v on the pedal, are so subtle that it is neglible. In studio you might benefit from it, a little, a tiny bit, but that's all.
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What I discovered is that if you change between the 9v and 18v, there's few seconds of havoc in the actual sound the first time you engage it. Almost like it chokes and then cleans it up having getting the circuitry accustomed to the new voltage. I tried to stay silent when switching it on, so it had some relay time to adapt, but no, it had to have a signal first. All subsequent on-offs were clean. Maybe some kind of buffer thing going on. This signal is quite ugly and loud for a second or two so take care and be vigilant.