driven, then the output impedance should be less than 1k to preserve output signal. This is a easily achieved  goal with a Cathode Follower.

Cathode Followers sound bad, don't they?
Not necessarily is the short answer. Admittedly, a poorly designed or misused cathode follower will certainly harm the transfer of signal, for example, using a 12AX7 based Cathode Follower to drive a 600 feed. A good rule of thumb is to outfit the Cathode Follower with a cathode resistor at least one half, if not one fifth, the value of the load resistance being driven and to run it at an idle current equal to at least twice to five times the desired peak current into the load. So for a 12AX7 based Cathode Follower with a cathode resistor of 100k and an idle current of 1 mA, the correct load might be a 500k volume potentiometer.

Once again, a Cathode Follower need not sound bad and if properly designed, it can sound as good as any Grounded Cathode amplifier. A quick re-read of September's Circuit of the Month provides more detailed information on transconductance and current's role in determining the sound quality of a Cathode Follower.

Where are the coupling caps?
The quick answer is that it does not need any. As the input to the circuit is at ground potential, no coupling capacitor is needed here. At the output, the resistor separates the cathode from the load and shifts the bias voltage up from the output, which is at ground potential.

This trick consists of a Cathode Follower that is loaded at its output by a compliant current source, that is a current source that does not have a predetermined quiescent current. What is constant about it is that it strives to maintain a DC ground potential input regardless of the current flowing through it. Is it then really a current source? In AC terms, yes; in DC, no. It offers a very high impedance (roughly, 1 meg) to any AC signal it sees and in this respect it is identical to a typical current source. In DC terms it works to adjust its quiescent current until its input is zeroed at ground potential (0 volts) over time (roughly, 3 Hz). This piece of magic is the result of a DC servo loop that is wrapped around the input of the current source and the output of the OP AMP. If the input moves toward the positive over a long period of time, that net DC drift is fed into the non-inverting input of the OP AMP, which causes its output to go positive. This positive voltage will further drive the MOSFET into greater conduction, which will pull the output towards negative. On the other hand, if the output moves toward the negative over a long period of time, that net DC drift is fed into the same non-inverting input of the OP amplifier, which causes its output to go negative. This negative voltage will move the MOSFET into less conduction, which will move the output towards positive.

The subtlety here is that the MOSFET is within the DC feedback loop, but outside the AC feedback loop of the OP AMP. As the time constant of the RC network made up of the two 1 meg resistors in parallel and the .1 µf capacitor is so long that no music can fall into it, the OP AMP presents a virtually constant DC voltage to the gate of the MOSFET. This steady voltage sets the amount of current that flows through the MOSFET; if the tube's idle current drifts over time, the OP AMP's  output will drift with it. While responding to the music signal, the Cathode Follower's output voltage and current will vary, but variation is far too fast to register at the input of the OP AMP and will be ignored. (If the OP AMP's is extended to include the source of the MOSFET, then the OP AMP would respond to the change in current through the Cathode Follower. Not a good idea.)

The 150k resistor that connects from the B+ voltage to the output is there to give the compliant current source a current path in the absence of a tube in its socket or at startup when the tube has yet to conduct any current.