Hello Ernest,
DSK has the right of it.
The early versions of the SlimServer firmware did not invert the polarity of the output signal to take the op-amp into account. By removing the op-amp the "normal" polarity was restored.
This was pointed out to the developers of the SlimServer software and later software builds corrected this.
The latest versions 6.5.0 and on, feature a 100 step attenuator. This software and firmware does sound pretty good, but you do have to reverse the polarity of speaker wires, if you are using an analog modded SB. A stock SB would work just fine.
If you care to try out any of the earlier versions of the Slimserver software or firmware, you may have to reverse the speaker polarity with the stock unit.
I hope I haven't confused you completely.

BTW Ernest, I tried to answer your PM but I received a message your in-box is full. Perhaps you should e-mail me. The old e-mail address I have for you doesn't seem to be able to go through