I am using a bass plugin called trillian, great plugin. I am using the double bass that comes with it in my recordings where the setting are one acoustic solo guitar, two acoustic rhythm guitars, and then the double bass.
The problem with the bass plugin though is that in some speakers, the speakers begin to “wobble”, “flap” and make a buzzing sound in rhythm with the bass. I think you understand the phenomena.

I am wondering if this maybe occurs because the double bass plugin goes down very low in frequencies?
Is there a general “rule” or something about how you want to EQ, or put low cut filter to prevent this phenomena from happening in some speakers?
Maybe cut everything below a specific frequency?
I still want to keep the bass “fat”, because of the setting its in.

Hoping for advices!

Double Bass? I cannot recall that patch. Can you tell us what the actual name of the patch is?

I don’t recall any sort of “buzzing” of this sort on any of the patches, but what you can do is check to make sure all of the FX are off. All of the patches use the FX slots.

Also, as you allude to, some speakers cannot handle very low frequencies, so what you are hearing could be second and third harmonic distortions. In some ways this is a good thing, depending. It could be the only way to get a very deep bass to cut thru a mix on smaller speakers with a higher native cutoff frequency.

edit: you may not want to cut the bass so low that it becomes thin on larger monitors, so it is a decision/compromise to make for sure.

You can try lowering some dB above the 100 Hz to see if it is really sub bass distortion, a simple paragraphic equalizer can help with this.