@carlsbad2 . Yes, I did feel that there were improvements on my first (and only) listen and I had a happy grin, but my system has changed a bit over a short period with boxes and cables, making real comparisons and unbiased judgements a bit tricky.
Note that the two boxes that died have 220V written on the back and I was feeding them 238 - 244, so more than 10% over. I also noted that one of them, a C/J 350 power amp, is now running 20+ degC cooler than it was before the step down/voltage regulator was installed (i.e. thermometer reading over the cooling fins).
I wouldn't completely rule out bigger power spikes as being part, or all of the cause, but adjusting the voltage, still seemed like a prudent thing to do. Otherwise, I would probably have gone for something like a Puritan Audio PSM 156 SE, which people are saying good things about elsewhere.