By disconnected, do you mean you removed the interconnect cables?
If yes, then power issue. If not, you are getting crosstalk from the preamp, though it is usually thought of as a Left to Right thing, it's also possible to get it from one input pair to another.
If yes, then power issue. If not, you are getting crosstalk from the preamp, though it is usually thought of as a Left to Right thing, it's also possible to get it from one input pair to another.