One of the best DIY single-driver/fullrange speakers is the Hammer Dynamics Super12's. The super12's use a 12" driver w/ a supertweeter above 10kHz. Because of it's design and cabinet size, the Super's rock w/ terrific bass, something not often found in single-drivers with a HUGE cabinet (bass relfex) or horn. Beyond that, the Super12 is supposed to have great clarity, delicasy, coherence, imaging, etc.
The drivers and directions cost $650 or so and the cablinet is extremely easy to build.
Read Dick Olsher's review on Enjoy The music:
There is a is a section dedicated to the Super12's on The Single Driver Website, which is an unbelievable source for information. Their forum is awesome, lots of serious experts to help out the beginners.