Check your instrument cable and make sure it isn't compromised. Also, if you are using a standard instrument cable directly into the Alesis, make sure the Alesis deals with a TS plug correctly. If you have a DI box or amp head in between, either of these could be the issue as well. And finally but perhaps firstly, the pickups on the bass itself might be the issue. Hiss is unlikely to be the Alesis despite my dislike of the Multimix line and it certainly isn't Sonar.