Breaking up the Hard Pan Sub-Soil
The soil below the raised bed should be broken up, I use a digging fork to break this hared pan up . keep working the fork as deep as you can making several pases over the bed in both directions "don't kill yourself here just take your time. stick the fork as deep as possible and rock the handle back and fourth to break the soil no lifting needed just break it up.
If you have a problem with Moles now is the time to lay some wire in the bottom of your raised beds or they will play Havoc on your plant root system.
Raised Bed Soil Mix
My bed mix is 50 % good top soil 30% composted horse manure and 20% wood and oak leaf compost. along with three shovels of per-lite. Good Luck GT