I have always used a dedicated extra sub bass synth to layer below my double basses. But I’ve heard some composers use a sub bass insert effect that generates a sub bass following the double bass performance.
What do you prefer personally and why? 
If I want realism, I tend to just use EQ (dynamic if necessary) to boost the existing bass if necessary.
When going beyond that, it depends on the context, tempo etc. For expressive “pseudorealism,” a generating the sub from the existing audio might be better, but for a tighter and more controlled low end, especially when going for a truly massive orchestral sound (bass gets messy with lots of instruments and big reverb…), I sometimes just cut most of the original bass, and use synth to manually add a sub layer instead.
Example, where I replaced the strings/brass low end with a two oscillator sub synth, playing octaves, with some saturation and (IIRC, dynamic) EQ. It can be tricky to get even bass on all notes, without boominess, which is where the EQ comes in. The reason to use dynamic is that if it’s more than just a pure sine wave, “anti-boom EQ” might take the punch out of the notes that don’t need that EQ, but with a dynamic band, you can selectively suppress boomy notes.
I believe it just depends on what sounds better really – what feels better. I just bought the Sub-Synth generator from Plugin-Alliance, need to try it, but what I have heart so far was quite cool.
However I like to be more in “control” in my productions, so I guess I would prefer more the extra layer sub-bass, so I can still make automations, using effects, side-chaining, etc.
Does it make sense?
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Yes I like the extra control with separate sub bass synth track too. I also automate level with mod wheel so I can follow the dynamic curve of the string basses.
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Did you get bx subsynth - not tried it on orchestral stuff but it’s amazing on synthwave, synth pop stuff.
You can make it in your face or just to add a bit of spice to yout low end - can apply it stereo or mid and make your sub mono etc - cool plugin
Yeah exactly, bx_sub_synth…will try it out soon! Will be interesting on which sources it will work best besides synths…
Actually, I think there is a strong case for “cheating” with synthetic sub bass, even in pure orchestral tracks: When instruments are played in the same space, they will resonate together and “phase lock” to some extent - sometimes so strongly that cellos demonstrate wolf tones they don’t actually have, and similar weirdness. (Feeling how strongly these instruments resonate when played, this is not very surprising at all…)
Since separately recorded samples will obviously not affect each other in this way, we may have to pull off some tricks to simulate it, to get more realistic results.
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Of course, if pure orchestral sound is what you desire, then this is not an issue at all. 
I am heavily inspired by hybrid orchestral music like Hans Zimmer, heavy metal bands, and electronica. I can layer 808 booms below orchestral percussion, sine wave sub below the low strings etc. and I love both the end resulting sound and the great control I get of the mix.
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Think it would work amazingly well for hybrid or trailer stuff but I think if you want extra low end to your strings and brass should work really well - have to try it out
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