I'm currently trying to understand Impedance matching and I'm a bit stuck on a few things. I've been trying to connect my headphone amp to the outputs on my DAC, but when I do I get distortion which sounds like something overloading. In the meantime, I've just placed my pre-amp in between (connecting the tape out to the headphone amp) but I'd like to remove it from the chain if possible.

Headphone amp is a Schiit Asgard and I was told by one of the engineers that it has an input impedance of 100k ohm. My DAC is an Arcam D33 and the manual states it has an output impedance of 47 ohm. From my understanding, it seems this should be okay as the amp has a higher impedance. My Arcam pre is a C31 and the manual says it has <50 ohm output impedance. Any reason as to why it works fine with the pre in the chain but not without it?

My other query is regarding passives. I'd like to try one but I'm not sure if my amp would match well. My amps have a quoted input impedance of 22k ohm, is that high enough for a passive?