I want to know what the disadvantages of direct coupling are,
particularly if anyone knows details of the implementation in my Line 2.
If direct coupling was the bee's knees, wouldn't all high end gear do
We make two preamps with direct-coupled outputs.
This preamp is not entirely direct coupled- it has a direct-coupled output. The advantage is increased transparency across the entire audio spectrum and bass impact as good as anything out there. This is because the coupling capacitor that usually causes preamps the most trouble is the one at the output; it has to be a fairly large value in order to drive solid state amps without loss of bass. This requires a large value to avoid phase shift; the result is inductance that will color the sound regardless of the type of coupling cap used.
It is also advantageous that the preamp is not entirely direct-coupled! That can lead to additional IMD due to modulation of the power supplies.
The disadvantage might be the complexity of the circuit and what is needed to allow the direct coupling to happen. In this case one of the disadvantages is a protection circuit with a relay (we avoided that in our preamps by having a circuit that can't produce large DC voltages should something go wrong- we have a patent on that process) so that an amplifier or speaker is not damaged, especially during warm-up. If the DC servo is not properly designed it can contribute to the sound of the preamp overall; in this case that does not seem to be an issue.