Sub amp plate that cross over at less that 30hz?

I have seen lots of sub amp plates, but all them seem to cross over at 40hz, but not lower. Can anyone point me in the direction of a sub amp plate that will cross over below 40hz?
Now that you mention it, I have never seen one that crossed over below 40.
The 500 and 1000 watt plates, available from Partsexpress, have variable Xover from 30-200hz and variable EQ from 18-80hz.
What sub would you cross over to at 30hz.You're talking one octave if you have a world class sub.
revel B15 sub has cross over points at 30,40,50,60,70,80.ook up revel speaker web site for full details