I have just replied and thought many other people could be wondering the same thing, as I did prior to making a purchase some time ago.
The detailed specs for the piano can be found on Yamaha's website. For those who are less interested in the numbers and specs, following is a summary of my responses to the questions I was asked earlier:
- Before I purchased, I had trialled the P-155 for touch and sound alongside some high end Kawai and Roland digital pianos and it stacked up easily as well (and in the case of the Kawai MP6 which is generally at a significantly higher price point, the P-155 sounded and felt better, more realistic, under the fingers).
- Many years ago I held the belief that keyboards with internal speakers were "low end", not as professional as those you have to plug in to external speakers. While sometimes a truism, this is not a solid rule. The quality of the internal speakers in the P-155 is fine, and part of the appeal for me with this model was the ability to sit down and just turn it on and play. Having played with synths in a studio environment for years, it is refreshing and more natural to have no need to put headphones on, turn speakers on, wait for the piano to boot up etc. It was closer to the experience of playing a real piano.
- Something you wouldn't think of and most people wouldn't think to mention - when you use a keyboard of some substance which has internal speakers, because the sound is created inside the keyboard there is a slight vibration you can feel through the keys as you play them - a sensation which feels a little more natural like the resonance generated by the strings of an acoustic piano.
- Like a wine, a car, or a restaurant meal, some people will judge an instrument's quality or lack thereof by price tag alone. This is a shame. Interestingly though, the Yamaha P-155 which retails in Australia for $2,199 actually uses the same AWM Dynamic Stereo Sampling sound engine as the Yamaha Modus H11 which retails for $18,999+. In other words, the wrapper may be different but the sound you're paying for is exactly the same.
As I did prior to purchasing, listening to and comparing clips of various piano performances on YouTube is a good way to get a feel for the quality of sound and sensitivity you can reproduce with various digital pianos. Hear the Yamaha P-155 put through it's paces further here.