There are Dual Concentric Drivers that are extremely efficient.
These are designs able to substantially impress and many options for how the Single Driver is used.
An Open Baffle through to a Transmission Line are options.
I use a DC TL Speaker with 5 Watt Output SET 300b and just love the moment that is created.
An uncomplicated OB design should not cost much more than the Drivers Only @ approx' $450 - $800
Add $500ish if a more complex Cabinet becomes of interest.
Have a look at Joseph Crowe he recently commission produced Don Sachs Speakers, of which Don states "never has better been ecperienced" or words similar?