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 it and MDF; I went for quality MDF for the cabinet. All joints use a 1/4" rabbit, for strength, along with a center brace.

Unlike the plan, I opted to make the bottom panel of the speaker removable; I found distributing the insulation using the speaker's mounting hole and bottom access was more than adequate.  This will also allow me to wrap the entire speaker in white oak veneer, which will then be ebonized.