Anyone every tried Audio Nirvana single driver speakers???

Maybe start with something good, but inexpensive.

http://www.wildburroaudio.com/speakers.php

I heard the "Non-K" versions in rectangular shaped open baffles and thought that they sounded pretty good/balanced without any type of correction network.

I've seen them sold with oval/egg shaped baffles online, but don't recall where.

DeKay

I don't know AN, but my current office system speakers are the Fostex based Brine 2000.  I've previously owned the Vaughn Cabernet's, an "augmented" Fostex designed with powered base driver and ribbon tweeter.  Not quite the large driver you are contemplating, but:

Pros are bang for buck, simplicity, and comparatively easy DIY build (unless you get into really funky cabinets).  Also have advantages for nearfield listening (hence their home in my office rig), and many designs work well with low power amplification.

Cons are, as you say lows; cabinet fixes, like transmission lines, can result in "tubby" bass.  But my main complaint is different, I feel like I hear something awkward around the the transition to upper mids, highs.
My hunch is that many people will end up preferring multidriver designs, but I think single driver is a great thing to explore, esp. for DIY.
You might look at ZU speakers, too: a good company that uses big full range drivers.