If the DAW were doing all calculations using 16-integer, then yes, once the meter went over 0dbfs into the red, on a channel or master, it would actually be clipping, and would actually sound awful. With 64-bit or 32-bit floating point internal architecture, you actually have loads of headroom above 0db.
I mean, isn't clipping essentially when a volume is calculated that is above the maximum allowable value? Why would a plugin ever output a value that high? It depends too on the patch, a fat huge pad with fast attack and slower release is going to be way louder than a tiny bell sound that has a totally different ADSR component. Patches also varied in output level quite a bit and if you were going on stage live often some levelling up patches was required beforehand so there were no surprises. It is not a lot different in a way from hardware synths.
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There are other points from within the synth programming too that determine the output level, so in some cases you can leave the output level set and alter the patch level further back in the synth programming architecture.
There does not seem to be much of any standardising going on with these things. Others can be right in the ball park too. Good news though is mostly if a virtual instrument is super loud and you do crank it right down to a normal level, there wont be any distortion present and none from that point on either. Others are just slamming though and sometimes I have to turn the main output volume on the synth to even as low as 9 o clock in order to get the right rms level leaving the synth. You can boost the output by +6 and +12 dB and I find I have to use the +6 dB setting often and also raise the channel fader too. As I was saying Korg Wavestation is just basically quiet and I mean quiet. It is quite weird in a way that the output levels from many virtual instruments do vary so much. Serum or Synthmaster on the other hand will be super loud with some presets. Korg Wavestation is very quiet and you have to boost the output by either 6 or 12 db to get a decent level. They all need tweaking in this area for sure. As long as presets are not changed they will usually stay at the level you have set. Many virtual instruments though often will not remember the setting of their main output volume once another preset has been selected so keep your eye on that. Do this before rendering as well to ensure a correct rendered level. From that point on you won't have any issues. in the inserts and simply set the volume on the virtual instrument itself for the correct VU reading.
So once a sound is selected on your chosen virtual instrument just insert the meter directly after the instrument e.g. Put it into K system mode so that it shows around 0 dB VU at the chosen ref level. There are third party VU meters that do this and also you can use the built in Level Meter provided by Studio One. You set up the VU meter such that it shows 0 dB VU at the chosen digital ref level. It is neither the fault of the instruments or Studio One. Some of mine barely move the meter and others are simply slamming the output. Many virtual instruments produce a wide range of levels for some reason.