null - микрофон без окраски (нейтральный)
none - вообще нет (т.е. ты слышишь только микрофон, с которым делался импульс.
На практике установка null позволяет немного имитировать разность частот при передвижении микрофона дальше или ближе от динамика (ручка Proximity в каб-блоке)
Из WIKI
"The "None" and "Null" settings disable mic coloration. A mic is still involved though, because the IRs themselves are always captured with microphones. Even when a neutral mic was used to capture, such as an Earthworks mic. When capturing IRs, the mic is most often placed very close to the speaker, so the result is a close-mic'd tone. Still, selecting "none" is a good way to prevent adding additional eq-ing to the tone.
The Proximity parameter simulates the proximity of the modeled mic to the speaker. Higher numbers translate to the mic being closer to the speaker (nearfield). Lower numbers translate to the mic being further away from the source, with the lowest number providing far-field coloration. Proximity only works when a mic model is selected, including Null.
The Proximity Frequency parameter allows tuning the frequency range over which the proximity effect occurs."