A sub underpins everything - can give the system some scale, sounding much bigger than it is. But it can’t make up for speakers that lack in quality. Would you spend more on the 5/7/9 speakers that are doing most of the work and producing direction information, ambience, music, dialogue etc etc, or a single box that is covering a frequency range of only 60-100Hz?
I’m not against spending big on the sub/subs, but it needs to be in context with the similar to some suppliers suggesting speaker systems worth £5k-10k with a £1k AV receiver (when they don’t sell mainstream AV receivers, or know they’re not going to make any money on the AV receiver).