I've run into this myself, and described the symptoms, what I did about them and my final conclusions on the post called "More piezo stuff - high order networks, etc." Sadly, in some situations, there is no passive solution that will damp the system and/or isolate the tweeter enough to prevent oscillation and maintain acceptable frequency response and system sensitivity.I'm afraid you may be out of luck with that amp. Try a few things more before we make this conclusion:
1. Run one woofer and one tweeter.
2. Run the pair of woofers in series.
3. Try a different amplifier.
4. Use a different tweeter, but still use a piezo. Try KSN-1038's instead of KSN-1005's.
5. If the guitar amp uses tubes, you might try a different output transformer.