Here are a couple articles written by an audio engineer, I thought were a pretty interesting read. Part 1 "Compromise and Approximation" is about ported speakers. http://www.soundonsound.com/sos/oct00/articles/monitor.htm
Actually, I liked part 2 much better, especially the section on thermal compression in passive speakers.
crossover filter circuits in passive speakers are dependent for their response accuracy on the input impedance presented by the drive unit. As the voice-coil resistance increases with temperature, the effective filter response's shape can stray very far from that intended and can introduce all sorts of errors in the system response. These thermal compression effects are notoriously difficult to predict or tie down -- they are entirely signal and signal-history dependent, for a start.
I also liked the part about off-axis response, particularly that of large drivers that are crossed over high, using low slopes. It made me think of fellow audiofools who spend countless hours worrying about break-in and countless dollars on cables trying to correct speakers that are performing exactly as they were designed. Let me put it another way, if you have a two-way speaker with a large woofer crossed over to a super-tweeter or to a normal tweeter at a high frequency, the large driver will most likely not have good dispersion, no matter how long you give it to "break-in". It also explains what can happen if you have a tweeter crossed at too low.
Find out a little about the off-axis response of your monitors. Don't necessarily mistrust a published specification but treat it as the marketing material it almost certainly is. You can work out much of what you need to know simply by looking. If you have monitors with a large bass/mid driver (say, 200mm or more) and a high crossover frequency (above 3kHz) you can be pretty certain that, however flat the axial frequency response, the off-axis response won't be. Conversely if your monitors have a smaller bass/mid unit and a lower crossover frequency they'll in all probability be better behaved off-axis...
Here is Part 2:http://www.soundonsound.com/sos/nov00/articles/ustandingmons.htm