I've read a lot of different things from different people.
Some say that the bass can go over the -10db line (on the IXL freq analyzer)
Others say a 1-2 DB below the kick drum.
My advice would be to listen to how the sub sounds on a big system that can go all the way down to the 40hz range. Or just use a freq analyzer and compare to noisia tunes or whatever. Skrillex has nice sub levels too on the Scatta tune.
I personaly found the ideal peak DB of sub bass to be exactly 3db higher than kick drum when high energetic dancefloor tune is being made.
but I was in studio with one of world wide famous jump up dnb producent and he had digitaly emulated analog type distortion on sub bass that was -3db and kick that was hitting that 0db edge even when put down by 8 db,.... and it sounded incredibely fat
depends on the tune and what i think sounds good in the overall mix. when i first start workings on the sub tho i usually stick on a compressor that will keep it at -10 db, this will probably change as the song progresses tho. i always find -10 is pretty loud tho, kinda dance floor level maybe.
if your subs taking up a lot of head room then just turn it down although if that sacrifices the power you need from it then side chainings always an option.