Don't think my preamp likes feeding signal to two seperate places.

Just discovered that my system sounded noticeably better when I disconnected the interconnects that feed my subwoofer crossover unit.  Simply having them connected, micro detail diminishes. Treble gets a little less extended. Is this what is meant as an impedance mismatch? It's obviously affecting the fidelity arriving to the main stereo amp.  Now I'm stuck with having better bass or better micro detail and sounstaging. Is there  anything I can do? I'm bummed out. The main stereo amp is fed by balanced out from the pre. The subwoofer crossover unit is fed by rca out from the pre. It seems like the preamp doesn't like feeding two things at once. This is a really upsetting discovery. It's not a huge difference, but noticable to me.  Now the system sounds absolutely incredible, but now I don't have any sub bass. 


ARC  LS 17 preamp-----main out rca to paradigm x30 sub crossover------adcom GFA555se (as subwoofer Amp)
 
Main output XLR to ARC  DS450

is this a limitation to the ARC ls17 circuitry? Would the preamps up the range not have this phenomenon? I know it's not arcs hi end model, but I wouldn't expect this from a unit at this price range. I wouldn't think I would have to have this compromise. 

Any thoughts?