The sound drops off db wise when the tone isn't super low. Example, if you know the song i put on by young jeezy, there is 4 bass tones. The shiva really hits on the 4th, but the first 3 aren't very hard hitting. Some of the bass notes, like the ones in Paper planes by MIA make the sub sound like a snare drum, not like it should sound. By upper base i actually just meant everything except the real lows. I use mac's... unfortunately. I don't have an spl meter.
2) Not sure if the reciever does have bass management, it has setting to change the db's of the sub. I have the phase switch off.. auto on on.. the other switch set to crossover.. the freq knob is set all the way up. And the volume set to whatever will make it not bottom out.
3) My main speakers are, for front and left.. Polk S10 Towers, and for the Center a Polk Csi3. The surround are in walls from monoprice.com. I plan on getting a new reciever in the next couple months for i can get the sub to work with it for now.
The people over at avs think it is the freq of the box.. here is my thread over there: