A big part of my 300B SET Amplifier project, is developing a pair of speakers suitable to be driven effectively from 6-8 watts. 
By far the best option is a full range driver, as opposed to separate 2 way or 3 way designs with invariable crossover losses. 
A horn loaded approach was also rejected because of space concerns. 

The business side of the 300B amplifier

I finally settled on Markaudio Alpair 7.3's that I found on the used market, from a very reputable seller.


These, in the right enclosure can be highly efficient, while still offering good bass performance.  Scott Lindgren,
a Markaudio partner has developed a variety of enclosure designs for specific  drivers.  I chose the Pensil 7.3

This is a quarter wave transmission line, which means the length of the cabinet works out to one
quarter the wavelength of the resonant frequency of the driver.  The diagram calls for a 7.5 square inch
rectangular port.  I will instead, provide a 3.10" circular port on the inward side of each speaker near
the lower front.

a = pi*r2   so solving for r  r= √(a/pi) we get r=1.5451

Although the plans call for 3/4" multi ply, such as baltic birch, a little investigation found a very
interesting youtube video measuring both.  As a result, I went for quality MDF for the cabinet, planning
on wrapping the entire cabinet with ebonized white oak.  All joints use a 1/4" rabbit, for strength,
along with a center brace.