I am a big proponent of doing all your sound shaping before it gets to the speakers and not relying on the speakers to add character to the sound - because you have far more control earlier in the signal chain.
That means a hi-fi approach to speakers - flat, full range response. Which, in turn, means a three way system with separate bass, mid, treble drivers and a good internal crossover - a la Acme or Acugroove.
Two full range apeakers is also a very flexible setup - use one for smaller gigs, both for big ones. They're interchangable in the event of a malfunction (provides graceful degradation in a live setting). In a pinch you can use them as a mini pa.
If the bass from such a setup isn't gut slamming enough for you, ad a subwoofer crossed over very low.
David B.