Maybe this is due to terminology subjectiveness 😀
In my view, ModernU can be called "muffled" because it does not have the thunderous and long-lasting bass notes of grand pianos. Using the same engine and sampling team, compare the basses of VSL Bosendorfer Imperial vs the bass from VSL Bosendorfer Upright, or the bass of ModernU to the bass of Ravenscroft274. To many people (me, included) that have no experience with uprights (or grands for that matter) and that relate to piano only from listening to audio recordings, ModernU (or VSL Bos Upright) possesses a tone that does not sound familiar, and that feeling is particularly uncanny in the bass notes.
On the other hand, the Pianoteq "muffleness" is something that affects all pianoteq models and in all regions (particularly in the middle octaves). In all pianoteq models, the bass is overwhelming, the middle muffled/nasal, and the upper octaves are piercing and shrill. Within the pianoteq models, I find the U4 less resonant in the upper octaves with a simpler harmonic content compared to other pianoteq grand models (Petrof Mistral, for example), but the bass notes are very grand like (almost unrealistic when compared with the sampled uprights that I know).
Of course, this is all subjective... If anyone could pinpoint what exactly could be done better in pianoteq, it would have been done by now and the sampled pianos would be history...