Efficiency is determined by the size of the speaker driver and the size and design of the enclosure. True efficiency is a balanced frequency response from at least 50Hz on up. Generally, this means that most 8" drivers will produce 50Hz at approximately 90db at one watt in a bass reflex cabinet. A 10" will do 93db at one watt and a 12" about 96db at one watt. A horn loaded speaker will produce more bass but the cabinet will be quite large.
Therefore, if a bookshelf speaker with a driver less than 8" has a published efficiency rating above 92db, it will most likely have a rising midrange and sound bright and tinny.
Therefore, if a bookshelf speaker with a driver less than 8" has a published efficiency rating above 92db, it will most likely have a rising midrange and sound bright and tinny.