Weird crackling from speakers when preamp connected

I've got an MC275 hooked up to a VTL 2.5 preamp, when I reconnected it after some maintenance, whenever I turn the preamp on then the amp, once they both warm up, the crackling starts.

I hooked up the amp to my DAC and had no issues with crackling, swapped tubes, interconnects and power cord, different inputs/outputs, same crackling issue when using preamp but no issue when connected to the DAC as a preamp.

Sonus Faber Olymica !, MC275 Amp, VTL 2.5 preamp, Auralic Vega DAC, nordost interconnects/speaker cables, panamax power conditioner.

My last step is to see if I can either borrow a tube preamp or buy a cheap one to test it.

Any suggestions are welcome as I'm running out of ideas.