I dont know whether a premium subscription uses a higher quality audio files, I always try and find the 1080 version of any track though, as there's such a huge difference in quality. Most of the time there is such a version available, and this is with no subscription.
Installing and adding uBlock Origin to your browser will block all youtube ads (and all other annoying or potentially harmful pop ups) for no cost. It is sometimes a cat and mouse game and once or twice youtube has caught up with uBlock, and blocks the content unless you turn the adblock off, but 99% of the time, uBlock are ahead, and you get to watch with no ads. I've only had issues for a day or 2, only a couple of times, in perhaps a year or more of use.
The adverts drove me fucking insane before first using a VPN, and when that no longer worked I switched to a VPN plus uBlock origin. I don't think you need to have both though, and just uBlock should work fine.
It'll save you £15 a month or w/e the cost of a subscription is.