ACR QUESTIONS CTLR ABOUT ALT CLRNC THAT THEY HAVE READ BACK AND FIND TO BE TOO LOW FOR ADEQUATE TERRAIN CLRNC. CTLR HAD MADE NO COMMENT ABOUT THE IMPROPER READBACK.

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WHILE ON ATC VECTORS FOR THE ILS RWY 31 AT HIBBING; MN (HIB) THE CTLR ISSUED US A DSCNT CLRNC. I READ BACK 'DSND TO 2600 FT;' WHICH IS WHAT I THOUGHT THE CTLR SAID. THE FO QUESTIONED THE ALT AS WE STARTED DOWN. WE CHKED THE MSA ON THE CHART AND REALIZED IT IS 3400 FT. HE SAID THAT IT SOUNDED TOO LOW FOR A VECTOR AND I AGREED. I ASKED THE CTLR TO VERIFY THE ALT AND HE CORRECTED US AND SAID IT WAS 3600 FT. WE ONLY DSNDED TO 4000 FT MSL WHEN WE CLARIFIED THE ALT. THE CTLR MISSED OUR READBACK AND WE COULD HAVE DSNDED DANGEROUSLY CLOSE TO THE TERRAIN. I DIDN'T NOTICE AT FIRST AS I HAD ONLY SLEPT 3 HRS THAT NIGHT. WE WERE ON A CONTINUOS DUTY OVERNIGHT AND HAD BEEN ON FOR 11 HRS 50 MINS. HAD I BEEN BETTER RESTED I MAY HAVE CAUGHT THE ERROR SOONER. STILL; THE FAA DECISION TO NOT HAVE TO CORRECT MISTAKES BY CTLRS IS WRONG. CTLRS ARE NOT REQUIRED TO VERIFY READBACKS; AND AS IN THIS CASE CAN HAVE THE POTENTIAL FOR DISASTER. THIS COULD HAVE BEEN AVOIDED IF WE WERE RESTED AND THE CTLR CORRECTED OUR WRONG READBACK.