i had no idea hifi amps were subjected to same precaution as guitar amps, when i turned on my SET amp without speakers connected to it. the amp is Spark 743D, and has slow start indicator- red then green. the amp went to green after usual warm up period. a minute later the 300b tubes suddenly went bright blue then the amp went back to red- protection mode. i turned off the amp immediately when i saw that. there was no smell of any kind and the transformers were cool to the touch. but now the amp sounds veiled and muddy- everything sounding like an AM radio broadcast. did i fry the 300b tubes? oh boy.. please let me know.