This one is a bit of a stumper. From what I can tell, the 1.4V setting equates to 33.1dB of gain and appears to be the default position. This knowledge base entry from Crown's site suggests that the 26dB setting is intended for pre-amps with an output gain of 2.5V or higher. I believe that the 990's output gain is more along the lines of 1V, which would suggest leaving the switch in the default "1.4V" position. If you're still unsure (and I know I'm not exactly certain aobut this myself), Outlaw support or Crown support might be able to help.