Aiyah Iansoh, joke at that time of the night...can't get it! Looks like the joke's on me!
OK ghn4ever, I'll try to repeat myself again - in simpler terms.
The only way you can get the sound out from the RD300SX is the audio output (the 2 jacks you see behind your RD300SX labelled "Output R, L(mono)" and the 3 pin cannon outputs (labelled "Balanced Out R, L"). The one labelled "Phones" also transmit audio signals - of which you connect your headphones. There is no other way you can get sound out from your keyboard.
The midi outputs you see at the back (labelled "midi in, out 1, out 2 and thru") with the 5 pins does
not transmit any sound or audio signals. It doesn't matter what you connect it to - it just does not transmit any sound. It transmit
midi messages, which is
not sound.
Midi 101. We use midi cables to connect to: 1. other musical electronic musical instruments or 2. PC via a midi interface. What this does is that it sends messages which will "talk" to the other instrument or PC, very much like networking computers. For example using Iansoh's example, if we connect a cable from your RD300SX midi out to your P120 midi in, when you play the keys from your RD300SX, the messages (ie what note you play, how long you hold the notes, how hard you hit the notes etc) will be transmitted to the P120. Mind you, these are not audio signals. What these messages then do is to trigger the same notes, same duration etc to your P120. Your P120 reads this signals, then plays exactly the same notes, but this time with it's own sound. On a PC, we use midi to do what we call sequencing, which is really programming or recording what we play into the PC (with appropriate software). The software will record the midi signals (I repeat, not sound) which can then trigger sounds from any other electronic instruments you connect it to. This will get complicated for you at this stage, and you don't have to go into it. The USB port you see functions the same as the midi ports - it only transmit midi signals, NOT SOUND. We connect that to the PC to transmit midi signals.
So simple answer - no. You will not get any sound if you connect the midi or USB cable to the computer or speakers. It's not about getting better sound - you will not get ANY sound. I hope this clears up some confusion.
As for your Bose, I think you should be quite safe. The power of the amp exceeds your piano. So your amp should be able to take the audio out from your digital piano. Just start soft, then slowly increase the volume. If the wattage is the other way round, then there's danger of damaging your speakers. You'll not damage your digital piano since you are connecting audio from your keyboard to your speakers, not the other way around. Nothing is sent ifrom your speakers into your keyboard (except it's own power supply). As for Creative speakers, you can try to see if the sound is OK for you. I wouldn't recommend it as the sound is not well-defined enough.
Does this clear things up? I know to most of us, this is really foundational. But I can understand to a beginner, it can get extremely confusing. Anyway, we don't mind helping you step by step to clear up the confusion. Just don't call me Prof...
