This is an '05 model Ampeg SVT 3 pro, as far as Im concerned, the tubes have never been changed on this amp. Its my first amp with tubes, and I was wondering what the signs of tube replacement are. There is always a hiss on the amp (but I have Bright and Ultra Highs set on, aswell as treble pretty high), but it isnt the problem. When I powered the amp this morning, I got a weird Am radio sound from it. Does this mean I need to replace tubes?