On the right, my old amp, a wonderful-sounding valve (tube) design, output-tranformerless (OTL), and on the left my new, solid state class D model.
Why change? The OTL consumes 450 watts of mains power to deliver 6 watts of audio (1.3% efficient), which in these days of global warming is obscene. The class D amp is by contrast 88% efficient. And dare I say it? I think the class D might even sound better, too.