Liquidity is an interesting concept, and to understand it requires some understanding of the technology behind Nebula. Simply speaking, liquidity is a parameter that controls the way in which impulse responses are mixed together in the time domain. It is implemented via smooth technology created by Acustica Audio. The liquidity fader controls the fades between these impulse responses. High liquidity levels can bring out phase issues and too low liquidity levels can create little clicks or noise on some presets emulations. The slider is present so you can adjust the amount of liquidity to match the desired result. Typically, keeping the liquidity level low will result in a more realistic emulation of sampled equipment.
I would leave this parameter alone (along with the attack/release controls on preamp emulations).