The top tube functions no differently from the N-channel MOSFET in the previous example. And if a DC servo is used, it too must maintain its connections to its positive negative and negative power supply rails. On the other hand, if fixed bias is used, making the top tube cutoff when the bottom fuse blows, requires accepting some compromise. Cutting off the top tube can only be done by decreasing its grid voltage, something not easily achieved with just passive components and the present topology. A good starting point is connecting the outside resistors that set the top tubes bias to the top power supply rail and the bottom rail.
    The assumption here is that when the bottom fuse blows, the negative rail will be so unloaded that its voltage will increase, forcing the bias down with it. This downward voltage swing working with the speaker's DCR should help limit the top tube's current conduction. (The speaker resistance functions like a cathode resistor in a cathode biased amplifier, as any voltage developed across it will serve to further turn off the tube by making its cathode more

positive to its grid.) If the idle current can be brought down to 500 mA, the speaker will probably be safe.  A better solution might be to use a simple active circuit to slam the grid voltage negatively when the bottom fuse blows. A high voltage transistor can easily be referenced to a voltage divider that under normal operating conditions does not turn on the transistor, but when the bottom fuse opens, increases its division point to where the base is sufficiently positive to trigger conduction. This trick is accomplished by using the collapsing voltage (that is moving closer to ground potential) of the bottom tubes cathode when its connection to negative rail is severed.
    If self bias is used on the output stage, then much of our worries disappear. The blowing of the bottom fuse severs the cathode's connection to its bias resistor, which results in the voltage developed across the this resistor falling. Here then is our means of increasing the negative bias on the top tube. Anchoring the voltage divider resistor string to cathode resistor, means that the bias will fall when the bottom fuse goes.