Subs are rarely a bad idea, and are usually well received.
It looks like you've picked most of the low hanging fruit, but assuming your speakers are well designed with good quality drivers, but built to a price point, it's very possible the crossover components are ripe for an upgrade. Upgrading the caps and cheap cast resistors to non-inductive wire wound types can be fairly easy, fairly cheap, and fairly audible. It won't change the overall character of the speaker, but could definitely improve clarity (and things associated with that....ie: soundstage, etc.)