I'm currently building a Fostex single full range driver bass reflex speaker. This speaker has been well documented at other speaker building forums, so I'm aware of its strengths and shortcomings.
I'm not much of an experimenter and thus would only stick with proven designs (Proac 2.5 clones come to mind) or ones that I've personally heard. Speaker design with crossovers is an art form. Synergy between drivers and crossovers and the tuning of the enclosure is difficult, even with math and computers. Speaker driver tolerances and matching are also a big consideration. The voicing of the speaker is a very time consuming endeavor. Most of the speaker diyers that I've talked to tell me that most of their experiments were disasters. And even their successes were not quite as good as what is commercially available when they factored in their labor (costs). I think that full range speakers are a lot easier to deal with primarily since you do not have to deal with the crossover. And there are a lot of really great designs that are simple enough that most of us could handle.
In spite of all of the pitfalls, I find speaker designing fascinating. I enjoy reading about what other diyers have done. Keep us posted.
TWL built Voigt pipes with Lowthers and is very pleased with them. Do a search and you can read about it.