Hi. I'm having a problem with my DSL5CR. When plugged direct into the amp and switched to the Classic gain channel it works perfectly. However, when switched to the Ultra gain channel, the guitar is very faint and what can be heard is a weak-fuzzy sound. When I first noticed the problem, I thought it was a case of the tubes needing to warm up, as after 30 seconds the level would gradually increase, unfortunately the problem has got worse. Could it be the pre-amp tubes at fault?