Power to the Pack Low Flow Solenoids of the flow control valves is removed when the Eng Bleed Switches are turned off.
If you had both engine bleed switches OFF, then you would want the packs to supply a larger volume of air since the APU cannot supply as much as the engine bleed air can.
You could switch the Pack switches to high mode, but that is why they are kept in Automatic....so it can be done for you when necessary.
Why would you switch off the Engine Bleed switches anyway?
Dual Bleed problems (not Dual bleed light)?