I own a Jcm 2000 Dsl 100 Marshall Tube amp, everytime I turn it up pass 2 it begins to lose high end and sound less piercing, like it totally changes tone with the loudness, because when it is 2 or lower you get a nice crunch high end presence piercing sound thats sounds great, but I turn it up louder it loses it. This is on Distortion BTW. Is there something wrong with my Marshall Head? And how do I know if the tubes or a tube needs to be changed? I have little knowledge with tube amps