Last time I repaired a varimu making pops ( generally coming form unbalanced transfo or unbalanced tubes), it was a modified attack and release.
Generaly, the time constant is too short or the circuit impedance is too low.
As I saw on the photos, the machine is " customised".
Try to supress the beautifull mods and start from here.
Your limiter will be certainly working fine.
But remember that inside voltages can be lethal !!!
Olivier Bolling