I don't think so. Let's say in this way: Pianoteq creates the sound, sends it to the sound card, finally the sound card converts the digital sounds to analog, amplifies it and you get it as an analog signal from the headphones Jack. The fidelity of this signal depends on the quality of your PC components. Laptos usually have good audio interfaces, but among desktop PCs, at least until some years ago, it was usual to find horrible sound chips.
By the other side, the SPIDF output (Sony/Philips Digital Interface Format) transmits digital data through a coaxial cable or a optical fiber, some studio monitors have digital inputs, the most common devices with this interface are home theaters.
Specially with optic fiber, the signal is immune to noise, and the conversion to analog is done at amplifier / speaker. So if the digital to analog converter of the speaker is better than the one your pc has inside, the sound will be better, but at this point I'm a bit suspicious about how fair a general comparation would be. The main point is that to use the digital ouptut, we need a speaker system with digital input.