I find the spring time, just after the thaw and just before the bines really start to grow is a good opportunity to manicure the crown.
As some have suggested you can dig out the crown and split it... my experience suggests a much smaller harvest than the previous year while it re establishes itself in the ground. I would do this every 4 or 5 years or so.
You can also get in there with some cutters and a little hand shovel and thin them out each spring too. This will help you manage the bed they're in a little better as the year wears on, its also a good opportunity to take some rhizome cutting to propagate or give to friends.
Either way, expect a lot of digging and cutting.
I have 2 giant cascade crowns in a 4 X 8 X 2 ft bed as well - 5 or 6 years old - which are badly in need of attention. i figure i have about month to decide how much effort i wanna put into 'em.