Hi Javelin...
If you get to around 1500' before you select the packs on, by then the outflow valve will have been trying to pressurize, and will have fully closed...thus the ear-popping surge when you turn the packs on.
What we used to do in that situation, was to initially select the outflow valve to manual, drive it about half open, then reselect it to auto, and immediately select one of the packs on.
The outflow valve will again drive towards closed, but with a pack operating, it will settle at a position that establishes the correct ROC for the cabin...when that has occurred, select the second pack on, which will only cause a minor blip on the cabin ROC indicator.
Then all is OK
Cheers