It seems the problem is solved, after a long reading of the MIDI implementation document. There are some controls, which are numbered 0 and 32 respectively, 00H and 20H in hexadecimal, as is used in the MIDI implementation document for the PX-5S. It talks about something called Bank Select MSB. MSB stands for most significant bit and represents some binary number that has to do with some activity in the piano, in this case selecting a bank. The MSB for the Grand Piano Concert sound is 64. According to the MIDI document, I had to enter the LSB, least significant bit, into control 32. That one was a bit harder to find out. I set the MIDI-OX program up with the PX-5S hooked up, switched to every tone on zone 1, from 00 (the grand piano) to the troublesome GM Piano 1, which is number 17 and logged all activity. The MIDI-OX registers the MSB and LSB, which I found out to be 40 and 00 in hex for all the tones, or 64 (just as it says in the tone list of the tutorial) and 0 in decimal, respectively. To sum it up, by setting ctrl0 = 64 and ctrl32 = 0, Notation Composer stopped playing the strings on top of the piano sound. Thanks for all your help, hope this experience of mine will be of use to someone who gets in similar trouble, but I actually hope noone will. At least I learned more about MIDI!