Marty,
Great suggestion to use headphones to test for acoustic feedback.
Jed,
If you don't have a way to listen via headphones with the speakers off, try playing the suspect passages with volume set fairly low. Listen to the L speaker with your ear very close to the speaker. If the distortion only occurs at higher room volumes then acoustic feedback is likely the culprit.
Great suggestion to use headphones to test for acoustic feedback.
Jed,
If you don't have a way to listen via headphones with the speakers off, try playing the suspect passages with volume set fairly low. Listen to the L speaker with your ear very close to the speaker. If the distortion only occurs at higher room volumes then acoustic feedback is likely the culprit.