Hi Hugo,
Before you made the changes to the Preset, did you change it back to the DEFAULT one in the Boost theme General settings page? If not then that's probably why it's not working now.
I'm assuming you added the Preset via the Boost > General settings > Preset file loader?
Try turning OFF Theme Developer Mode as that will only slow things down and lose all the cache you actually need. Every time you add any code to the Boost theme settings pages, be it General or Advanced settings, once you save those changes the code is regenerated, so when the Theme Design Mode its not needed at this stage, but it is useful if you were making changes to CORE php code or changing CSS files then Theme Designer Mode works OK in those situations.
Anyway, after you have purged all caches you should reset your Browser cache using the Ctrl + F5 keys on your keyboard,that forces a reload of the page. So providing that you have put all the things you changed back as they were originally, done the browser reset, if things are still not right, then just Log out and take a rest. Otherwise you'll end up going round in circles.
Cheers
Mary