So, my take on this might deviate slightly from what has been contributed so far. I have owned many Audio Research amplifiers over the years. On two occasions, with two separate amplifiers (D70MKII and VS110); the same pyrotechnics you experienced took place. Both occasions sacrificed components (resistors) in circuit and required a trip to Audio Research for repair.
On both occasions I was living in a costal community, in a home without air conditioning and it was summer. Neither one of these amplifiers could possibly put out the heat of your Ref750 amplifiers. One key takeaway for me; if you’re using an Audio Research vacuum tube amplifier(s), make darn sure the listening room is plenty cool.
Currently enjoying a VSI75 in an air-conditioned environment…