Please pardon me if this is a dumb question. This is my first tube amp and my first Marshall. Recently I purchased a used Marshall class 5 combo locally. I was drawn by the lower power feature and the tones I was hearing via YouTube. The previous owner replaced the stock tubes with 5751, a 12AY7 - 6072, and a EL84. Last night I plugged into the amp for the first time with my Firebird. I had the amp dimed out and used my guitar to control the volume. I could not get any break up or crunch. It was amazingly clear. What tubes do I need to get a good crunch/classic rock sound?