I promise this isn't a "tone is in the fingers" comment, because it isn’t, but in Lemmy's case most of his sound comes from the bass and the way he plays it. His actual amp sound, and distortion, changed a lot over the years - you've got the classic early neck pickup tone to his later oft-quoted bridge pickup "bass off, mids full, treble off" which to me sounds like chewing tin foil.
People also forget that those EQ settings will only work on a Marshall Superbass, so the bass and treble are effectively flat rather than fully cut.
The Okko seems like a decent pedal but the distortion was only one part of Lemmy's sound.