Five or ten minutes is not tube warmup (which is more like 15sec) but rather the warmup of other circuitry. That may include bad solder joints.
It's often possible to hunt such thermal problems down using a bottle of cold spray (which is specifically used for such diagnosis). Of course, you don't want to reach the hot tubes with that in order not to cause the glass to crack.
Thermal problems are really time-intensive to hunt down. The usual culprit will be a capacitor with instable dielectric.
If your sound disappears in a hard manner, like switched off with a switch, it is more likely a bad solder joint as opposed to a bad capacitor than when it just abates gently.