"What is the difference...?"
Between the Tortech and the Signal SU/DU at 2000 watts, about 20 kg and several hundred dollars.
An autotransformer only has one winding and offers no isolation and no filtration. They are more efficient per weight and cost. If you want a packaged unit, that may be the only option available.
The mass of bigger core with dual/quad windings does give it some magnetic stability, if I'm using that term properly.
Bandwidth is not an issue for power transformers, unlike audio. They only have to provide 50 or 60 Hz. In fact, the narrower the better.
As far as degradation, there are various opinions and experiences and I can't account for each implementation although I've seen some that were just wrong. I've got several oversized isolation and center-tapped (don't ask) transformers and haven't heard any degradation, just a bit less noise. Not a big proponent of using them for amps because of the way a class AB amp draws power in spikes, but as long they're sufficiently oversized, it shouldn't hurt.
Be aware too that transformers can have a mechanical hum that can be unpredictable to each installation and environment.
