To run 4xEL84 in series the heater supply voltage will be around 25Vac. Each EL84 and each 12AX7 with parallel filaments wants to see 6.3V. But the supply voltage will only split evenly between the 4 tubes if the hot filament resistances are equal. The EL84 filament corresponds to 8.3 Ohm but the parallel 12AX7 filament to 21 Ohm. So each 12AX7 will get a much higher portion of the supply voltage than the EL84s.
To even out heater voltages you need to wire appropriate resistors (Rx) in parallel with the 12AX7s' heaters, so that 21Ohm//Rx = 8.3 Ohm. I get Rx = 14 Ohm. Wattage rating at least 5W, wirewound.