Originally Posted by Student
It's a general rule that the most efficient exchange of signal between to objects happens when their impedances are closely matched.
If you mean "input impedance = source impedance", then you're wrong. That would mean precisely 6dB of signal loss (or: 6dB more noise). The only reason to use that would be *very* long cable runs (talking a few kilometres or more here) when reflections become an issue. Phone lines, for example.
In recording, we need voltage (U), not power (P). This is achieved by having the input impedance at least 10 times the source impedance, preferrably more. A typical SDC mic has somewhat like 35 ohms, a typical mic pre has about 5k.
To the OP: Both versions have been done. A keyboard usually would have line level outputs, so it would rather go to the line input. In the end, it's the old story: If it sounds good, it is good.