Hey everyone! I recently have been having trouble with the ashdown abm head. in the beginning of rehearsal, it sounds great and has plenty of volume. after a bit of playing the volume will drop and i have to really crank it to get the volume at a good level. I have already opened it up and switched out the preamp tube and it still drops in volume. Do you think there is something I can do before I take it in to a tech? any advice will help, or if you can point me in the right direction i would appreciate it. Thank you!