I've never gone higher than 38Hz and it was a little too boomy for my liking. 35Hz has been the best sounding out of the several I've built/heard.
And Louisiana CRX seems to have some experience with DD subs, so if he says 35Hz I'd listen //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif