@gte357s - besides the arcing problem, are you enjoying your ANK monoblocks? Are these driving your Spatial X5s? Are yours the parallel SET amps? What 300Bs are you using?
I'm also using mine to drive OB speakers - GR Research NX-Oticas - which also have powered woofers. Mine aren't as efficient as the X5s though, at 92.5db. But I'm going to replace the NX-Oticas with GR Line Force speakers (hopefully) in the next few months - just waiting for the cabinets to be completed and I can start assembling them. The Line Forces are 99db/w efficient so the 300B amps should have plenty of power.
btw - it's only the interstage and output transformers that are C-core. The mains transformer and choke are more conventional (EI-core I believe).