2. A super low output impedance is essential, isn't it?
In order to avoid insertion loss and frequency loss, a low output impedance is absolutely necessary isn't it? Well, it depends. Now, consider that cheap IC's such as the LM741 have amazingly low output impedances because of the high feedback ratios they run; yet they can't drive low impedance loads because they are current output limited. Yet a discrete transistor line amplifier with a higher output impedance (because of less feedback) and a greater output current capability may drive the same low impedance load extremely well. So which was the crucial factor: low output impedance or high current output?
Of course, if the proceeding stage represents an extremely low load impedance, a low output impedance will be needed just to preserve signal level, but not necessarily to preserve bandwidth, as any cable capacitance would tend be effectively countervailed by the load's own low impedance. No, the real threat to bandwidth comes from high impedance loads, which are bogged down by the high capacitance (because of long interconnects and the Miller Effect of the input tube of the amplifier); and when this capacitance cannot be charged and discharged quickly enough, poor bandwidth results. The key words in the previous sentence were "charged" and "discharged." Charging a capacitor quickly requires current. The quicker the charging, the greater the current flow. The formula is a simple one: Current = Slew Rate * Capacitance or I = SR * C, where slew rate refers to the amount of voltage that must be developed within a certain amount of time. Therefore, in order to guarantee wide bandwidth, the line stage must be capable of delivering a fairly high current at its output.   

3. Isn't phase inversion bad?
Some line stages invert the signal polarity and are to be avoided at all costs...right? No, unless you can't reverse the positive/negative connections of the speaker cable to the power amplifier. Line stage phase inversion just needs a screwdriver to fix. If the line amplifier inverts the phase and the power amplifier doesn't, then invert the speaker's phase. If the line amplifier inverts the phase and the power amplifier also inverts, then don't invert the speaker's phase. Really, it's that simple.

4. Which circuit to use?
A little bit of gain, a fairly low output impedance, and a robust output current ability are all design objectives for our line stage amplifier so far. Additional design objectives for a line stage are a low input capacitance and a good power supply rejection ratio (PSRR) figure. The Cathode Follower and the White Cathode Follower have low output impedances, but no gain. The Cascode amplifier has a low input capacitance, but an extremely high output impedance. The Grounded Cathode amplifier offers a good power supply rejection ratio (PSRR) figure, but a fairly high output impedance. The Common Cathode amplifier has many virtues, but its high output impedance (basically, 2 * rp) disqualifies it from consideration. Likewise, the Plate Follower meets many of the design objectives met, but its low input impedance makes it unsuitable for use with most volume controls, as its input impedance will shunt the potentiometer resistance. The Totem-Pole has a low output impedance and a good PSRR figure, but is complex and requires an internal coupling capacitor, which can prove both expensive and complicated to bias, and needs more tuning of values than most imagine. Once again, this leaves the Constant Current Draw Grounded Cathode (CCDGC) amplifier as a good candidate. Besides meeting all the previous design objectives, it would match last month's phono stage nicely.
Once again: the CCDGC amplifier consists of a Grounded Cathode amplifier directly cascaded into a Cathode Follower. Each triode sees the same cathode to plate voltage and the same load resistance and current draw. Both are in voltage phase, but not current phase. (As the grounded Cathode amplifier sees a positive going input signal, its plate current increases, which increases the voltage developed across the plate resistor, which in turn swings the plate voltage down. This downward swing is then cascaded into the grid of the Cathode Follower, which decreases the plate current to the same degree that the previous stage's current increased.) This results in the constant current draw feature of this