I am not a bass newb, but I have never been really knowledgeable of the electronic aspects of amps. I have an older SWR redhead, and I am experiencing a wierd effect. I hit a note, then shortly after can hear "crackling" interference in the signal. I have tried it from different AC outlets, and with a surge strip. Do you think it could be: The pre-amp tube? Speakers are blown? My guitar cable? The sound is not consistently distorted or crackly. It happens sporadically.