If you consider the amount of money and time typically invested for fancy components and high quality cabinetry, it makes sense to also invest in a calibrated measurement microphone. It is a must if you want to do it right (and want to be consistent with how much is invested in the other aspects of a loudspeaker build). As for measuring components: Speaker Workshop does an excellent job of it, plus it allows you to do Thiele/Small measurements, enclosure design, acoustic measurements and crossover design, with auto optimisation for frequency response and impedance. Plus it is free. The only barrier is that it only works with Windows versions up to XP (maybe even Vista)*. But even that is a minor issue if you still have an old PC lying in the garage; a dime a dozen off Gumtree if you don't. And SW has an excellent design example in its help section.
*I think I may have had limited succes with Win7 (or perhaps it was Vista) at some stage, but the Help section wouldn't run.
