I'll throw out my experience with the old HD's. Current box was around 8.5cubes @ 38Hz for 2 15's. I mostly listen to 80's metal, old school hip/hop & rap, and only play the newer stuff to demo it.
Box did hit the lows, but sounded sort of boomy on some songs, and was definitely more peaky. I extended the port and dropped tuning to 35Hz, so the box is around 8.25cubes now.
WAY better for kick drums when listening to metal, and on new stuff like the Akon/Snoop "I wanna F you" remix it flexes the rear window a good 1/2" or more. Couldn't tell you which tune was louder as I've never had it metered since it's just a DD, but the 35Hz tune is much better for the music I listen to.
For guys wanting to get as loud as possible and mostly playing newer rap, then 38-40Hz is probably right on...