If you require that kind of efficiency, you may want to take a look at some of the 10" Guitar speakers on the market made by Eminence and Tone Tubby. Some offer alnico magnets, some with hemp based cone material, most with high-ish Qts that many find desirable for OB. Not always the last word in detail or ruler flat response, but if done correctly, can offer a very nice presentation overall. I am running two different sets with only slightly more powerful push-pull tube amps (one EL-34 based and the other 6L6GC) and both sound very nice.