I’m curious to know what these issues are with direct-coupling are since I may go this path with the SLP-98.
The Cary employs direct-coupling but not at its output- there it uses a conventional coupling capacitor.
But to answer the question, direct-coupling issues are
1) the unit must not make DC that could damage an amp or speaker
2) the DC must be able to stabilize during warmup without damage to amp or speaker
3) This requires a servo control; the servo circuit must be stable at all input signal conditions and not impart any sound of its own
The tricky bit is actually 2) above, in many solutions a protection relay is used and the relay itself can contribute (or rob from; depends on how you look at it) to the sound. We patented a circuit that is reliable enough that a protection relay is not needed.
I may have overshot here- in case you were only asking about the direct-coupling used prior to the output of the Cary. If that is the case, the big issue is power supply stability, as the circuit, except for the output, has bandwidth to DC. Power supplies, even though they are a source of DC, have a timing constant that is something just shy of actual DC like a battery has. If the circuit can go lower than the power supply, low frequency instability can result. 'Motorboating' which is a problem in amps and preamps with failing power supplies, is an example of this phenomena.
