Following some advice from one of Australia's top Car Audio designers (FHRX Studios), I ended up sealing the under-seat enclosures & Dynamatting them.
The supposed ported enclosure the stock speakers are in, is a joke. It has 2 openings that could not possibly be tuned.
The supposed port does not go in to the B pillar, as there is sound insulating foam there.
Having a small, sealed enclosure should also assist in their power handling - when amplified.
Midbass sounds far more accurate (less boomy) in the sealed enclosures and I've fitted a pair of inductors which has removed the high-end superbly (thanks Jim!).
FYI - my X3 has 6.5" midbass - not 8"! The top looks 8" but the woofer itself is a 6.5"!
It's sounding pretty darn reasonable now, with the staging back up just above the centre of the dash (inductors worked wonders with this) - just need to sort out the loud mid's with some more EQ...