It must be tuned by expert hands. But before tuning, one must ALWAYS check the efficiency and regulation of the action and the tuning. Even an out-of-tune unison can make a note "shoot." Never use needles if you are not an expert.
I was just reading about the muffled sound: it becomes such if the strikes occur disproportionately, to the point of killing the hammer action. One must strike in a scientific and graduated manner. In this way, the timbre will always be precise across various dynamics. There are areas of compression and other areas of hammer expansion that must be respected. The hammer should be an iron fist in a thin velvet glove.
Very few technicians are also good voicers... The action, the tuning are ok... voicing is the most artistic part of the adjustment. There are voicing techniques to open up the sound more (see how Steinway does it, for example). The sound desired by the Manufacturer should not be "killed" with disproportionate voicing, but tamed: small touch-ups repeated over time to unify the timbre and correct harshness. A Yamaha must not sound like a Pleyel, otherwise it would result in being muffled (as you say); and vice versa!!!. Every instrument has characteristics given by its design but also by its intrinsic nature (that is, how it is born) and voicing should emphasize these characteristics rather than suppress them. Voicing does not mean softening, or at least not only that, but giving the hammer back the right zones of compression and expansion. The choice of hammer felt is also fundamental: if chosen well, it will need less voicing and therefore will preserve its specific peculiarities for longer.
All this was learned also from negative experiences on my pianos, as well as from the wise advice of Maestro Ferrarelli.
