Sub Bass Synths
The ultra low bass frequency support for instance on sustained (string) chords is very common in today’s film scores and one of the style defining traits of “modern Hollywood”. However, it is practically not achievable with acoustic instruments like Contrabasses or low brass. Also, EQing in the mix to boost these frequencies is usually not desired as it will also boost a lot of low noise rumble and distort the overall characteristics of the sound.
The usual way of adding the sensation of this “larger than life” orchestral exapansiveness is to double the bass line with a sub synth, either at pitch or in some cases even an octave below. The sound characteristics of that synth should be to have quite few overtones but a full round bottom end, so a sound that is close to a sine wave is what usually works best. Any synth that has too many overtones might be consciously audible in the mix which in this case should be rather avoided. Also, the attack of the synth sound should match the orchestra, so a too hard synth attack phase on otherwise sustained string chords will not sound great.
As usual a word of caution: while it sounds great and adds a lot of extra oomph to any orchestral recordings, as with any special device its use should be saved for when you actually need it. Doubling entire scores like this just out of principle will diminish the impact it has. Rather save this for the moments in your cues where you really need it to open up.