Adding two Cathode Followers solves both the problem of dissimilar plate voltages and the problem of  too high an output impedance. One Cathode Follower is used as a simple capacitance multiplier that locks the first triode's plate voltage to the DC value of the second triode's plate. The 1 meg resistor and .1 µF capacitor filter the AC component from the plate swings of the second triode's plate  The second Cathode Follower buffers the output. Since most small signal triodes come dual packaged in one envelope, adding the two Cathode Followers means only adding a second tube.

With Balanced Inputs
If used with balanced inputs, then a protection diode should be added. When the circuit is powered up, the diode is reverse biased and does not conduct and thus drops out of the circuit. But at turn-on when the cathodes are cold, the diode is forward biased and prevents the cathode from falling below one volt negative. This protects the cathode, as in the absence of the diode, the cathode would see a huge positive voltage on its grid while its cathode would be at the negative power supply rail voltage until it heated up sufficiently to emit electrons and conduct current.  The diode should not be used when the Common Cathode amplifier is used with an unbalanced input, as it will limit the maximum input voltage swing.

Which Tubes to Use?
Ah, the eternal question. One third of all the e-mail we receive asks. What if I can use a 6_ _ 7 or ECC_ _ _ instead? As long as the power dissipation is not exceeded, just about any dual triode will work. The question is how much gain is needed.

If high gain is needed, then the bottom tube could be a 12AX7, 12AT7, 5965, 6072, 5751, or a 6DJ8/6922 would give a medium amount of gain. If you prefer octals, then the 6SL7 will work or what would be even better is a 6SU7, which is identical to a 6SL7, but much better matched between triodes. If less gain is needed, then the trustworthy 5687 is a great choice, but very demanding in both idle and heater current draw.  Other choices are the 6FQ7, 12AU7, 7119, and the 12BH7. If you prefer octals, then the 6SN7, 6BL7 or even the 6BX7 would make a great line stage. Remember, the top and bottom tubes need not match, for example a 6SL7 on the bottom and a 6SN7 on top work great.