I've got a carvin R600 head that is 4, maybe almost 5 years old. It's a hybrid and has a single 12AX7 preamp tube in it which I have just recently noticed a little distortion in. It pretty much only happens if I have the volume on the bass and the gain turned up pretty high, but it has kind of a decent overdrive sound, and if I want to get rid of it I can just turn the knob over to solid state. My question is, does that mean it needs to be replaced soon? i.e. will the distortion rapidly get worse and worse or is there any possibility of hurting the amp by leaving a bad tube in? I suppose it won't hurt anything as long as I use only the solid state part, but since I haven't had to change a tube before I thought I'd ask about it. This is more a question of opinion, but how should I go about choosing a replacement tube? Should I just get another Carvin tube or would it be worthwile to experiment with some other type? I know there is a lot of personal preference when it comes to tube amps, but personally, I couldn't tell much difference between the tube sound and solid state sound until now (since it started distorting). That might be because of the hybrid preamp though, or the type of tube it had, or maybe I just haven't listened to it that closely.