"Quack" is the sound you get on a strat in positions 2 and 4. It's the out-of-phase sound of two single coil pickups combined. Another description would be "plucky", a la dire straits. To get that kind of sound, you really need single coils that are close together.
> HH provides treble and bassy stuff(neck)
> while an ssH pridides more mids with a few lows
You can't really generalize like that- depends on the type and combination of pickups, wood, etc.
Generally speaking, the more inductance a pickup has, the smaller the frequency bandwidth it has. (Inductance doesn't exactly equate to power, but you can think of it that way.) So louder humbuckers have more mids and less highs & lows. Single coils will have more highs and lows (as well as mids). When you combine pickups out of phase, the distance apart will determine what frequencies are cancelled out- the further apart, the more frequencies that will be cancelled (due to the difference in the string vibrations where the two pickups are located).
Hope that helps.