How old is the MC275? I had a similar issue with a pair of Ohm speakers and a Sansui receiver that a qualified technician could not replicate in his shop. Turned out to be the receiver. It was 30+ years old and needed to be recapped.
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.