That's the $64 question.
I believe scook may be spot on with his assessment.
Adam, did you get sorted out with Session Drummer 2? That will be your best sounding option for synths included with SHS6XL.
TTS-1 would also be a viable choice. Both are GM mapped, so the drums would be triggered correctly.
I would suggest going to Options>MIDI Devices, and deselect the Wavetable synth as an output, so it will not be available to SHS6.