Hey guys I figured out a method myself that works. All you have to do is open up any samples that u want to sustain/end when u hold down/let go of a key in the wav editor and set the loop points. Then in the channel settings window for the sample click "Use Loop Points". Now if you've set your loop markers in a good spot the note will sustain (i.e. keep looping) but also end at the moment you let go of the key...
I dunno if this is the best method but take it for what you will.
Oh and here is a tip for when setting loop markers in order to create a sample that will sustain (such as a one-shot bass hit for example). Now, most of us recognize how wav forms will start at the zero crossing, then go up, then come back down to the zero crossing, then dip below the zero crossing, then go back up to it (sounds more complicated than it is, im just describing the simple "wave" shape, and indeed it it shaped like a wave, up and down up and down etc.). And by the way, the zero-crossing is the point at which the wav line/amplitude meets with the horizontal line that runs through the wav.
Now the trick is to set your starting loop marker at the zero crossing of the first amplitude (i.e. line/hill of the wav going up, starting from the zero-crossing) but this doesn't neccessarily have to be at the very start of the wav file itself, just at the start of the point you want to loop. Now, place your end loop marker at the zero-crossing after the wav line has gone up, returned, gone down and returned TWICE (starting from where u placed your starting loop marker), making two cycles. Your sample should now sustain its note well, allowing you looped a good spot and placed the markers exactly on the zero-crossings.
To sum up:
1) Place first loop marker at zero-crossing of beginning of wav amplitude
2) Place end loop marker at zero-crossing after two wav cycles (up, return, down, return = one cycle)