http://www.linkwitzlab.com/frontiers.htm#F
you can also look about it on this site:
to answer your question,
1000/30=33, you have 6, GREAT
1000/20=50, you have 18, great imo, it falls below half a cycle.
in the linkwitz lab article, i skimmed it and noticed something about 50ms being percieved as an echo... from these numbers im assuming its a sealed box? or a smaller sub enclosure, my gut says you will be fine as far as group delay.