user: Question: Name the driver matched db for Joseph Grado Signature
Columns: ['Headphone Model', 'Headphone Class', 'Sensitivity (dB)', 'Impedance (Ohms)', 'Driver-matched dB', 'Construction', 'Earpads', 'Termination', 'Succeeded by']
Types: ['text', 'text', 'text', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: ['SR40', 'Unknown', '100', 32.0, 'Unknown', 'Plastic', 'Foam', '1/8" (3.5 mm) plug with 1/4" adaptor', 'iGrado']
SQL:

assistant: SELECT "Driver-matched dB" FROM "Table" WHERE "Headphone Class" = 'Joseph Grado Signature';