FO OF ACR LTT ACFT ALLOWED THE ACFT TO DSND BELOW ASSIGNED APCH VECTORING ALT DURING A TURN TO FINAL.

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ACR FLT # FROM MCI TO MHK WITH FO AS PF. WE WERE BEING RADAR VECTORED FOR THE ILS 3 APCH AT MHK WITH 6000 FT THE ASSIGNED ALT BY ZKC. THE LOC FREQ WAS IN USE ON BOTH CAPT AND FO RADIOS WITH MHK VOR IN STANDBY ON BOTH AND ALSO GIVING US DME READOUTS. AT 13.8 DME A DIFFERENT CTLR SUDDENLY CAME ON AND TOLD US TO INTERCEPT THE 13.0 DME ARC FOR THE ILS. IN THE TURN; THE FO AS PF DSNDED 450 FT BELOW THE ASSIGNED ALT AND THEN PROMPTLY RECOVERED. IT WAS APPARENT THE FO WAS TRYING TO DO TOO MUCH ALL AT THE SAME TIME; I.E.; TURNING TO INTERCEPT A RAPIDLY APCHING CLRNC LIMIT TRAVELING AT 250 KTS; SWITCHING THE FREQ FROM STANDBY TO USE AS WELL AS SWITCHING THE DME. AS THE PNF; I WOULD HAVE TUNED THE RADIOS FOR THE FO BUT WAS STILL RESPONDING TO ATC WHEN THE DSCNT OCCURRED. ALTHOUGH THE ALT BUST DID OCCUR; IT WAS CLRLY THE FAULT OF THE CTLR. IF THEY PROFESS TO ADHERING TO PRACTICES THAT PROMOTE AND MAINTAIN A SAFE AIR TFC SYS; THEY SHOULD NOT SPRING SURPRISES ONTO THE FLCS. WE WENT FROM BEING RADAR VECTORED FOR THE APCH BY A MALE CTLR TO BEING TOLD TO INTERCEPT THE ARC BY A FEMALE CTLR WHEN WE WERE UPON THAT 13 NM LIMIT WITH ABSOLUTELY NO FOREWARNING. EVIDENTLY; THERE WAS A SHIFT CHANGE BECAUSE THAT WAS THE VERY FIRST XMISSION BY THE FEMALE VOICE WHICH IN ITSELF CAUSED US TO DO A 'DOUBLE TAKE' AS WE WERE NOT ATTUNED TO HEARING THAT PARTICULAR VOICE GIVE US INSTRUCTIONS ON THIS LEG. WE HAD PREVIOUSLY BEEN INTO MHK THAT DAY AS FLT # AND WERE VECTORED FOR THE ILS 3 WITH NO INCIDENT. THE LATE ABRUPT DEP FROM NORMAL PROC BY THE CTLR FURTHER COMPOUNDED THE FLC'S ABILITY TO REACT ('TURN ON A DIME').