The right room.

From my experience, Stehno's suggestion is accurate.

All room dimension rules are made to be broken but going with a long rectangle as he suggests will give your speakers room to breath and allow them to reproduce even the deepest bass passages.

Pedrello,
One of my best friends has a room that's about 25 wide, 38 long and 12-14 high. A very large space, and his reference speaker is a medium large three way with 13" woofer.

The sound is excellent, although perhaps too low of an acoustical output for my taste with his current power. The new amps he has on order will likely bring this system into the realm of best of the best.

Point I'm making is larger is better, unless you have too small a speaker or too little power to get the room moving.

I agree with your comment about two way speakers. With only one crossover point they get closer to the ideal of NO crossover point. Your question of coherency is valid, fewer crossover points are not a guarantee of phase accuracy but are definitely a big factor.