First, the amp is not in the equation...at all. An 18 volt mod, for pickups and preamps that can handle it and claim, by the manufacturer (like EMG) to run with more headroom as such should not be any louder than running at 9 volts. It simply raises the ceiling before clipping occures within the pickup's own internal pre (such as EMG's) from the amount of magnetic signal taken from the vibrating strings. In other words, given everything but the voltage being equal, a given (hard) note that clips the pickup's internal pre at 9 volts may not clip it at 18 volts due to the higher ceiling available. It is not making them run hotter/louder.
After the pickups, if there is an equalization pre amp (Tone Pump, etc.), your tech is correct about one thing: it probably doesn't need the 18 volts. But unless you want to run a separate battery/power system for the pre (one battery for the pre, two for the pups), it's kind of along for the ride.
In a passive pickups scenario, unless the added equalization pre's manufacturer recommends it, an 18 volt mod is unneeded. This all has to do with the internals of the bass and has nothing to do with the amount of signal hitting the input stage of the amp. This is where your tech is way off base.
I've been doing 18 volt mods to almost all my EMG equiped basses (mostly Spectors

) for well over 25 years. I've never had one get louder from it....as they shouldn't.
Most of us here seem to think the 18 volt mod is about our equalizations pres (EMG, Tone Pumps, whatever). It is not.
It's about EMG recommending 18 volts for the pres inside it's active pickups. Most can even be run at 27 volts.
However, some manufacturers (Aguilar, etc.) actually recommend 18 volts for some of their equalization pre's.