AFTER COPYING AND READING BACK THEIR CLRNC; B737-300 FLT CREW IS ADVISED BY APCH CTLR THAT THEY WERE 500 FT LOWER THAN THE ASSIGNED ALT.

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WE WERE BEING VECTORED FOR RWY 13 IN VISUAL CONDITIONS WITH SOME SCATTERED TO BROKEN CLOUDS 2000 FT AGL. COMING FROM THE NE. I HAD BRIEFED THE ILS RWY 13 INCLUDING THE 1474 FT TWR ABOUT 3 MILES N OF THE OUTER MARKER APCH GAVE US A DESCENT OF 2000 FT. WE BOTH HEARD IT; SET IT; READ IT BACK; AND POINTED TO THE ALT WINDOW. I WAS CONFIGURING; SLOWING; AND LOOKING FOR THE ARPT. I POINTED OUT THE TWRS AT OUR 1 O'CLOCK POS AND COMMENTED THAT THEY HAD US RATHER LOW RELATIVE TO THEM. APCH ASKED US IF WE HAD THE FIELD IN SIGHT; BUT WE WERE PRETTY SHALLOW AND I COULD NOT SEE IT YET. THEY THEN ASKED US TO CONFIRM 2500 FT; AND ALTIMETER SETTING. I INITIATED A CLB TO 2500 FT SMOOTHLY XING THE OM TRANSITIONING TO A STABILIZED DSCNT HAVING SPOTTED THE ARPT AND BEING CLRED FOR THE VISUAL 1-2 MI OUTSIDE OF THE OM ON THE L BASE. CO-PLT AND I TALKED ABOUT IT AND ARE 99% SURE THAT OUR DSCNT CLRNC WAS TO 2000 FT. WE READ BACK 2000 FT SO THERE WAS A HEARBACK PROB AT THE LEAST. I SHOULD HAVE QUESTIONED THE ALT ASSIGNMENT SOONER AS THE PROC TURN ALT WAS 2600 FT; BUT WE WERE BEING VECTORED IN BASICALLY VISUAL CONDITIONS WITH A RATHER CLOSE TURN ON AND WERE RATHER BUSY. I WILL NOT ONLY INCLUDE THE LOM XING ALT IN MY BRIEFINGS AS I DO NOW; BUT THINK OF IT AS RELATIVE TO VECTORING AS WELL AS A GS XCHK WHEN INBOUND ON THE LOC. THINGS WERE STARTING TO NOT FEEL RIGHT AND WE WERE WATCHING THE OBSTRUCTIONS WHEN THE CTLR QUESTIONED US. WE SHOULD HAVE CAUGHT IT EARLIER.