If the loops are audio and not MIDI, it cannot be done.
Softsynths are not effects but are instruments. You cannot apply a softsynth to an existing audio track as an effect.
You can use a softsynth in your project by
adding a new MIDI track and new audio track to your project.
add the softsynth to the Fx bin in the audio track
assign the output of the midi track to the softsynth
record the new midi track or if the loops are MIDI, just copy the MIDI loop to the new track.
If you are looking for a synth type effect check out
http://frettedsynth.com/ If your bass loop are single notes, the "string sculptor" and "tripp lead" may be the effects you are looking for.
Alternately, there are a myriad of other effects that you can drop into the Fx bin to alter the sound of the loop.
post edited by scook - 2006/05/23 10:48:20