SX-1x80 series basically use some of the same parts, this was the introduction of Pioneer labeled IC's, which are hard or expensive to find. SX-1x50 series and earlier it was not so and they do not require as many work arounds if certain parts need replacing.
I'd stay away from a sx-1980, they are hugely expensive. So is a 1280 for that matter. Who actually needs that much power? Even so, if I needed a 300W/ch amp, i'd be using a separate for that!!