Well I was only comparing the 750 to the 900 so I'm not sure what you disagree with. To my ears the 900 was identical to taking a 750 and applying EQ to the bass to increase its quantity. Doing that causes boominess that begins to mask some of the bass texture and bass resolution and further recesses the mids. Essentially the 750 is a 900 with less bass quantity in exchange for more bass texture & bass resolution and less recessed mids. And that's why, unless your main requirement is bass *quantity*, the 750 is superior to the 900 IMO.
See this is where I'd completely disagree again.Obviously, there's no right answer here and everything is very subjective, but the Pro 900 isn't a headphone I'd call boomy in any way even with the monstrous amount of bass it can dish out. If you look at the specs of the drivers on the Ultrasone site, the Pro 750s have different drivers (different frequency response not due to different cups/damping etc) than the Pro 900s, they just both happen to be plated with Titanium. It's obviously a personal preference for which headphone one appreciates, but having heard both the 750 and the 900, I can confidently say that I don't believe either headphone has boomy bass. If bass quality
and quantity weren't important for me, I would have picked something more notorious around these forums. Also, another thing to note is that, sure, as the bass quantity increases the mids and highs do tend to get masked since in comparison to the large amount of bass, the other frequencies tend to take a back seat; that isn't the same thing as having loose and boomy bass. These two concepts are completely different in nature and a headphone (or speaker for that matter) can have tight, controlled bass while dishing out a very large quantity of it (albeit this is pretty rare, that is why the Pro 900s are highly touted by bassheads).
Make no mistake the 750 and the 900 do definitely sound similar and a lot of arguments can be made for or against if the 900 is worth the extra premium over the 750, but the 900 doesn't have the boomy bass that is normally associated with lower quality drivers that can't handle massive low end response. I've heard a lot from many people who really dislike the Pro 900, and a majority of their complaints are that these cans have recessed mids which is completely accurate (like you mentioned); it is almost impossible to have one piece of the frequency response be so exaggerated without overpowering the rest to some degree, but none of these people have claimed the 900 bass is loose, boomy, and uncontrolled. In fact, you're the first person. Again, obviously this is an opinion based discussion, but I just wanted to point out that having loose bass and overpowering the mids are different concepts which can and sometimes do (but not always) overlap.