digitech rp-7 tubes

i have a fairly used digitech rp-7 effects pedal.anyway i changed its tube some time ago with a marshal one (don't know what model exactly,when it comes to electronics and such i don't understand much) which, as i have noticed doesn't glow enough, at least not like the previous one.Could this mean its problematic or something?if the tube makes any difference in the sound, what tube would you reccomend for a metal style of sound?