Nebula cannot null because, like it's analog hardware counterparts, has variances. If you pass audio through a Nebula instance 2 times in a row, the resulting waveforms will not be identical. So when you reverse the phase of one, there will be aspects that will not null. That is also the behavior of hardware.
Giancarlo said something about static programs but I don't know what's that. When I started using Nebula a noticed a subtle difference on each pass (something I didn't expect from plug-ins back then) so I did a null test, investigated a bit and understood a bit how Nebula works so it became clear to me that making Nebula null with itself is very hard (and just luck, if it happens).
Now there are these "static programs", I wonder what they are... never heard of them.
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the answer is SOMETIMES. It depends on the specific template used for sampling. Dynamic behaviour is a variable like other one, you can't understand if a program is dynamic or not, and honestly it means nothing. A null test is a nonsense which is very trendy lately.
I add: nebula it's based on optimistic approaches: everything is dynamic. Then, if things don't change, it caches. Only the library developer knows how many samples are located in each single vector. It's possible he take a single sample when freq is 1khz and 300 samples if freq is 2khz. And even if he takes 300, maybe they are very similar and they appear as static, and maybe they pass a null test because you loose differences in noisefloor. Things should never be visible in a null test for being classified as good... it's possible the harmonic content doesn't null exactly, but it's already at -80db, in such case it matters but it's quiete invisible. It's like you have someone in neurovegetative state and you ask: is he alive? and you perform a null test using an hammer on his fingers...
In fact one of the earlier famous null test is from RhythmInMind.
A better way is to use VSTAnalyser to match EQ curves and load & hear them with your own ears.
But I found matching Nebula EQ's with regular PEQ's are pretty hard curve-wise. In harmonics view, some softwares tend to overdo with too much harmonics. Comepare FX with residual is not always meaningful. Which sounds good may depend on how your track sources sound.
A null test is a nonsense which is very trendy lately.
Amen to that. I've seen countless threads on other forums where people are trying to null different DAWs against each other. I mean, what's the point? Being able to null, doesn't make anyone a more skilled mixer, it doesn't enhance your musical talent, what it does is turning this week into the next week, LOL!