Our HumDinger test amplifier is an old stock Dyna ST-120 that has a conventional iron rectangular power transformer.
It is quiet, hum free, and still works well on our normal AC power here.
However when we give it our hum torture test, a electric drill plugged into the same outlet set and run the drill at part power, the old ST-120 hums and vibrates so badly it nearly jumps off the shelf.
Our HumDinger stops this hum completely, making the 120 immune from the DC the drill is putting on the AC power line.
So - - - your power transformer does not have to be a toroid to hum with low level DC on your power line.
An observations regarding this. Your small signal equipment may benefit from the HumDinger too. You might need to be close to them to hear the hum, but the waveform that a transformer affected by DC puts out is really really ugly, and a power conditioner that is not specifically designed to block DC won't help this at all.
One client bought two HumDingers to take care of his big mono amps, and then came back and ordered four more, one for each other piece of audio equipment he had in this system. He told us the HumDingers just made his system sound better.
We don't claim that, but with a HumDinger you will be getting cleaner electricity to your system for sure.
Heck, its only $140 and has a 30 day satisfaction guarantee, better deal than that $500 power cord that did nothing for DC at all.
Frank Van Alstine