Every Vienna Instrument comes with its own, individual volume. It depends solely on the recording gains of the samples (which is set to capture the sound with optimum level) and doesn't relate to an absolute reference.
"Natural Volume" is simply meant to bring the instruments back into their typical dynamic relation (... which isn't an absolute magnitude either). Of course this means that the headroom for the loudest instruments like the Tamtam has to be guaranteed, which makes the overall loudness quite soft as soon as you don't use those loud instruments. Simply raise the output volume of your mix, then, or bring up all individual players by the same amount.
... I'm not a fan of the "early reflection plus reverb tail"-approach to mixing and don't use it myself, so someone else will hopefully chime in with helpful information.
Kind regards & good night,
/Dietz - Vienna Symphonic Library