Guys,
my initial motivation originated from the observation that when I played the PT sound through the internal speakers, it still sounded slightly like from coming out of a box (actually difficult to describe verbally). This was especially for notes in the C4-C6 region, despite all my tuning efforts. This is actually better for the internal sound and I assume that Yamaha did tune the sound to the speaker. When then looking for a better loudspeaker for PT5, I came across those transducers (=exciters) offert by Visaton which are actually by cheap.
In my first experiments I tried to use both (tranducers and the internal speakers). But there was still this boxed sound to some extend. Then I worked only with the transducers. Putting one at the top of the piano, doesn't look good, but actually you can hear that the sound comes now from top of the piano. It is also important that the exciters are not on the same wooden slab. My last improvent happend when I closed the lid in the software model of U4 with PT5 (in the microphone tab). This really made a difference and the sound is now really natural.
I do have 8 Ohm transducers and with my (poor) amplifier I have to use maximum gain in all elements (PT5, audio interface and amplifier). I would say that the sound is now as loud as an acoustic upright, but definitely not louder (expect for the bass region, where it can get really loud). If you place the transducers at other places, you can hear some clank and rattle. Eventually this will also occur at the current positions, if I further increase the volume.
The tactile response is a very nice side feature, but it was not my intention. Thanks also Beto-Music for the NU1 image. Regarding the tactile response, I would say that the NU1 hit the sensors relatively hard. Eventually Yamaha could have used softer hammers, but maybe there are acoutic pianos which hit hard too? Unfortunately I have not too much experience with a grand action.
The internal sound is great, but with the PT5 the richness and complexity of the pinao is at least one level better. But I think I do not have to mention this here...
Thomas