The 26AQ8 is the same as the 6AQ8 with a different heater voltage and current rating. Both tubes are undervalued in audio applications. The 26AQ8's 26 volt heater voltage is only 14 volts shy of the rail voltages, which could be achieved by using a series resistor to drop the 14 volts. Because of the large voltage differential between the top and bottom triodes, the DC referencing of the heater is critical. Optimally in this circuit, the heater power supply should be reference to about -50 volts, which would split the difference between the cathodes. Attaching one end of the 26AQ8's heater to the -40 volt power supply rail is close to optimal. (The voltage dropping resistor would then attach to the other end of the heater and ground.)
One possible problem is the inability of the 26AQ8 to draw sufficient current to quickly charge the large input capacitance of the MOSFETs. Overcoming this potential problem will require using a different tube. The obvious choice is the one made by David, the 7370.