Tighten the bolts (or nuts if you don't have hurricane nuts) across from one another, going back and forth between them. Then tighten the other pair. Do not bend the speaker frame.
I don't know if that makes sense. Lets label the bolts clockwise starting at the top of the speaker or the right hand top side, if the speaker is installed at an angle. 1, 2, 3, 4. So you first tighten 1 and 3. Rather than tightening 1 all the way and then proceeding to 3, it is best to alternate between them so you tighten them both up at once. When you get near the appropriate tension, you can just do a quarter turn on one and then the other, alternating between them. When 1 and 3 are finished, repeat the same procedure on 2 and 4. This will help out immensely.
As for tightness, I have not really read a manual on this so I can't give a numerical value for how tight they should be. What I do know is that you are not to bend the speaker frame as this can damage the cone. Generally, the bolts will be tighter than finger tight but they will not be tight the way you put wheels on a car.
Hope this helps.
I think you'll enjoy the c90. I love my oversized 2 x 12 with a v30 and c90. Sounds very even!
