[IMGDEAD]http://i35.tinypic.com/fliyqx.jpg[/IMGDEAD]

For seamless synchronization between the woofer "W" and the tweeter "T" the distance from both acoustic centers "D" should be no greater than half the wavelength of the crossover frequency.

To illustrate a practical example I will use a very small tweeter - the Dayton ND20FB-4 Rear-Mount

Dayton ND20FB-4 Rear-Mount 3/4" Neodymium Dome Tweeter | Parts-Express.com

The impedance graph shows a peak at around 2kHz prompting for a crossover somewhere above 3kHz. To be safe and to simplify the calculation let's assume a crossover at 3440Hz. That leaves us with an acoustic centers distance D of 50mm. The overall diameter of the Dayton ND20FB is given as 35.7mm so the distance from edge to the center "dt" is about 18mm. Assuming no space wasted between the tweeter and woofer "ds", the resulting overall diameter of an appropriate woofer should be no greater than 64mm. That leaves all woofers greater than 2" out of the picture. Even the tiny 2.5" Peerless 830985 can't make it by 5mm.

Moving the crossover back to 3000Hz lets us choose from drivers with outer diameter less than 78mm which barely makes the Hi-Vi B3S the only reasonable choice at the expense of tweeter distortion.