We have also noticed the change in quality for newer MBP models. I have a MBP 15″ and recently had to have the left tweeter replaced as it blew about 1 year into normal (well, normal for me 😉 use. It seems Apple is taking the route of using signal processing to improve performance and they are still tweaking parameters.
Check out the MacBook with Touchbar profile in PROFILES tab / Built-in / Apple. This addresses the more fragile nature of the new speakers and delivers better overall performance. We are in the process of making a bunch of new profiles for new products, MacBooks included. I also suggest giving the Universal City Profiles a go as they work on a wide range of laptops and all provide good speaker protection and high-frequency response.