Unfortunately, the power transformer in this line stage is not center-tapped, which would have offer an easy means to creating a negative rail. The above circuit, however, will do the job. It is a effectively a voltage doubler that creates a negative power supply voltage. The extra negative voltage provided will allow a more optimal choice of operating voltages within the amplifier. The problem we faced in the earlier design was the danger of too great a voltage difference between the output tube's cathode and its heater. This forced us to accept a relatively low plate voltage for the 12AX7, as this voltage DC coupled to the 12AU7's grid.

     If the negative power supply prove too noisy, the common cathode resistor can be replaced with a 1N5305 FET current source. Grid stopper resistors have been liberally added. And a safety diode protects the cathode follower at turn-on. So what we have is remake that will sound much better and still work within the present limits of the line stage.
    On the other hand, if the power supply is much bolder than I have imagined, the number of potential circuit climbs. In fact, if the power supply is only marginally stronger, then using a high voltage negative power supply rail may be possible. 

      By freeing up the heater power supply, however, we can safely increase the 12AX7's plate voltage, as we can reference the heater power supply at +75 volts above ground; thus, splitting the voltage difference between the 12AX7 and 12AU7's cathodes. Shown below is the version with a high voltage bipolar power supply. The often recommended (in this journal at least) power supply noise reduction trick is implemented at the negative side of the power supply. The 75k resistor is bypassed to increase the amount of negative power supply noise into the 50k common cathode resistor.

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