CREATE TABLE table_1400 ("Headphone Model" CLOB, "Headphone Class" CLOB, "Sensitivity  ( dB ) " CLOB, "Impedance  ( Ohms ) " FLOAT, "Driver-matched dB" CLOB, "Construction" CLOB, "Earpads" CLOB, "Termination" CLOB, "Succeeded by" CLOB)

Name the driver matched db for joseph grado signature
SELECT "Driver-matched dB" FROM table_1400 WHERE "Headphone Class" = 'Joseph Grado Signature'
SELECT "Driver-matched dB" FROM "table_1400" WHERE "Headphone Class" = 'Joseph Grado Signature'