Very, very strange the divergence of opinions here. For a number of us, significant degradation of low end sound quality was obvious from the get-go and confirmed by careful testing, while others pipe up with comments like "better bass" and "sounds way less tinny now" that are hard for me to comprehend.
Three possibilities: (1) Maybe this isn't "just how the new ones sound" instead some of us received defective speaker units; (2) Maybe those saying things like "I love the bass! Though I didn't own any of the previous iPad Pros..." don't really know or remember how much better the 1st/2nd gen sounded; (3) Maybe this is just subjective and "everyone's ears are different", with some prioritizing high end and others low end sound.
For myself, I resonate with Jackfudge's comments quoted above, fearing Apple made "a VERY small and weak speaker" that's "too thin to produce a decent low end sound". The vibration issue is interesting... cranking up 3rd gen volume did produce appreciable bass but coupled with vibration that felt like it was hurting my ears. That matches another commentator who said "The last model had more bass? I thought that my new 12.9 had too much, it's vibrating in my hands from the bass output." The problem is bass QUALITY not quantity!