Detailed info sought on 987 sound package plus
 

I have a 987 with the upgraded (slightly) sound package plus, but not Bose. I've played with the base adjustment and the loud setting, and am wondering something that I haven't been able to find the anser to anywhere. Does anyone know what frequency at which the base adjustment is centered? It seems pretty high, which makes things a little boomy. On the other hand, the loud setting seems like it boosts the base down pretty low, and a little too much, so I can't find a combination of settings I like. It also seems very source dependent.
Thanks