You might consider some single driver speakers.
They make a nice match with 300b SET amps.
Lowther, AER, and Fostex make some very nice single drivers which can be loaded into a variety of cabinets. There are rear horn types, front horn types, front horn/rear horn types, Voigt Pipes(TQWP), and even bass reflex types that will work with these drivers.
They are high efficiency drivers, ranging from 95db to 102db efficiecy with 1 watt input. The cabinets can improve the efficiency also, depending on the type you select.
This totally eliminates crossovers, and provides "direct drive" of you speaker elements, with no intervening parts.
There are flaws in single driver speakers, just like there are flaws in multi-driver speakers. If you like the sound, and can live with the drawbacks(generally little bass below 45Hz), then they can be very satisfying. Some people also use them with subwoofers to augment the bottom end.
They are very viable for a low power SET amplifier, and can provide some assets that cannot be achieved with many other types of designs.
Some ideas would be Beauhorn, Lammhorn, Lowther Medallions, Hedlund Horn, Voigt Pipes, Oris Horns(front horn with subwoofer), and some others. You can do some research on this with a Google Search.