The welbourne labs link is about the most complete for low powered tube stuff I've seen. The fostex and lowther drivers are efficient and have had a loyal following for years, although I don't know how high-fidelity they are. The only other company that comes to mind is Lambda acoustics. It looks like they market their systems as pro-audio and home, but they specs look nice. 102 and 108 db sensitivities depending on options w/ only a 16" square horn mouth. But they'll set you back $5,000 or so for a pair. The horn operates 300hz to 20,000hz with less than 1% distortion even at 110db across the band. The designer is a rocket scientist.