How to Get Pristine Audio w/Keyboard?

I've got my Yamaha XS-7 plugged directly into my VS-700R.  The good news, I've got it to work.  The bad new is, there is a subtle hiss to an audio track.  When there is actual audio, for an example a piano line being played, the hiss is not heard.  However, when the actual audio stops but the track is extended out beyond what was played, I can hear that hiss.  When I reach the end of the audio track, the his abruptly stops.  

This might not be a big deal when there are multiple busy tracks.  However, if the multiple tracks are done with the keyboard, I am afraid the hiss would be compounded.

Also, if I have a project where I want soft, quiet music, there will be dead spots in the actual audio which would enable the listener to hear the hiss.  That'd be distracting.

I have worked with MIDI in the past and tracks I've done with MIDI, such as the Fantom and Cakewalk TTS, are very pristine and there is no hiss.  

Is there a way to eliminate this hiss?

I have similar issues with my Yamaha MO8: faint hiss and buzzing. It's too low to be noticeable in a live situation, but annoying for recording. The solution is S/PDIF, which bypasses the final stage of amplification in the synthesizer as well as audio cables, and thus the sources of noise. Recording via S/PDIF is essentially a file transfer - no analog noise, no analog-to-digital conversion.

Just remember to change your interface's clock source from Internal to S/PDIF before recording.

First thing is to look at gain staging - do you have the keyboard volume turned up to 11?  Or the input to your 700 high?

Next is cabling.

Then power.

Then it gets more difficult.

@

Besides S/PDIF, the XS-7 firewire option would give you multiple simultaneous digital channels.

In the analog realm: multiple approaches, starting with trouble shooting, as suggested above. Is the hiss equally bad on L & R channels?

Also the gain structure approach: Does the Yamaha have a master volume control, and does the hiss vary with that? Is there a setting of the master volume that maximizes SNR with track volume adjusted in Sonar? Can the patch volume in the synth be raised and the master volume be lowered? Keep the MIDI volume maximized and lower the corresponding audio track volume.

Also, there are ways to denoise any segment of audio where there is no music (noise gate/expander).