Single driver speakers. Are they worth considering ?

Only if they are field coil design

How about a pair of Omegas with a pair of monster subs? 

How about a pair of Omegas with a pair of monster subs?

I had the Omega Compact Hemp, Alnico Super 6 Monitors and Alnico XRS 8 all with dual Omega subs. Could never get proper integration until I used a Dspeaker Dual Core. The Dual Core helped to blend and manage the bass well but not perfect. I never used the X4 but have been told by others who own one, that it does a fantastic job. 

The best bass and sound top to bottom has been with my Charney Audio Companion Horns with Omega RS7 drivers. Charney doesn’t make the Companion with RS7s anymore. The Voxativ and AER drivers are so superior in every way. Hoping to unload my Companions soon and step up to the Voxativs. 

I would love to hear the Shindo Lafite.  I like the Latour (two-way field coil Shindo system).  I really like field coil drivers--they sound very dynamic and "alive."

While I would be totally satisfied with a Charney full range system, and perhaps the Voxativ back loaded full range system, I also think iopscrl (above) is correct that you get much of the advantages of such drivers while ameliorating the limitations of trying to deliver all the sound from one driver by using such drivers to cover a wide midrange.  I have heard the Jensen M-10 (13" field coil) with a tweeter, and the GIP Laboratory 4165 (12" field coil) with a tweeter to deliver the kind of dynamics and clarity that full range systems are known for, while being much smoother sounding and extended at frequency extremes.  

I have heard Western Electric 755 drivers used in single driver systems and in systems with a tweeter on top (only crossover is a first order high pass to the tweeter, the 755 runs full range) and much prefer having a tweeter in the system.  I think systems using the Western Electric 756 and 728 with tweeters are even better.