I must be a failed audiophile because I have no desire to spend £20,000 on a hifi system. Maybe £3000 to swap my DM10 speakers for the newer DM12 model but I'm not even particularly fussed about that either as I'm perfectly happy with my DM10s and have no desire to upgrade them. *unknw*
But if forced to spend £20,000 I would probably use it to fit out a custom built home cinema/games room. To start with I would spend about £2000 on a custom built ultra quiet gaming PC. Audio would be courtesy of a 5.1 surround sound speaker setup with AVI DM12 speakers all round. Maybe a large JL Audio subwoofer (or two) instead of the usual AVI subwoofer because they go deeper for the movies. As they are active speakers there would be no need to go overboard with the AV reciever/processor so only a few hundred pounds would be spent here, probably on a second hand one.
A largeish TV or projector, probably second hand again to make the most of the budget. Also some well chosen room acoustic treatment, some pretty furnishings and a few comfy chairs to finish it off. I think that lot would come to about £20,000.