I think the tube in my GBE 750 is about dead - when I use the tube preamp I frequently get significant drops in volume that are immediately remedied if I switch to the non-tube preamp. I never had to change a tube. my questions: does the volume drop on the tube preamp indicate a bad tube? can I change the tube myself? must I use the same kind/brand of tube, or can I change brands (and if I can change brands, what do you recommend)? thanks!