The sound output on Amazon’s Echo is fine for casual background music, but it’s not worth much for any critical listening. But now that both Google and Apple have speakers that can pump out some serious tunes, it seems Amazon is starting to take sound seriously. Pocket-Lint caught an Amazon UK listing showing off an unannounced Echo Sub – it seems someone posted it too early – listed at £75 (~ 99 USD, but gadget prices don’t usually follow exchange rates tightly).