I'll add that the preamp should be on when you make the test, but with no signal playing.
It also might be worth checking your source players for DC leaks. Depending on the preamp design, there is the possibility the preamp is fine but is passing DC originating elsewhere.
It also might be worth checking your source players for DC leaks. Depending on the preamp design, there is the possibility the preamp is fine but is passing DC originating elsewhere.