Hello,
I don't think there could be any loss of efficiency or whatever. You just need to make sure the sub will be balanced once on its feet. You might consider placing the sub with an angle from the floor so that the CG of the box gets more inside if you find balancing problems (am I saying that correctly :dontknow
.
Regarding the 3" on top or bottom I am not sure I understand as well (you might want to post a sketch??)... but I can say the lower the sub, the more you will benefit from coupling with the floor, and that might increase output... So it all depends.
I don't think there could be any loss of efficiency or whatever. You just need to make sure the sub will be balanced once on its feet. You might consider placing the sub with an angle from the floor so that the CG of the box gets more inside if you find balancing problems (am I saying that correctly :dontknow
Regarding the 3" on top or bottom I am not sure I understand as well (you might want to post a sketch??)... but I can say the lower the sub, the more you will benefit from coupling with the floor, and that might increase output... So it all depends.