Crackling sound - MesaBoogie SubRocket

I've got a ten plus year old mesa boogie subrocket amp. When the amp is on, you can hear a crackling sound through the speaker, and also through the headphones. I'm assuming one or more tubes has gone bad. I already replaced the two larger tubes to no avail. I bought one of the smaller pre-amp tubes SPAX7, but the problem persists.

Is this crackling sound a symptom of bad tubes, or is there something else going on? I don't feel like blowing $120+ on all new tubes if there is something more going on.