During arrival in IFR conditions, the pilot was vectored for an ILS runway 25 approach. At about 0909 PST, he was cleared for the approach and instructed to maintain 3000 ft until intercepting the localizer. A short time later, the controller instructed the pilot to contact the tower. At about that time, the controller noted that the aircraft's transponder reply was indicating about 1700 ft, approximately 1100 ft below the minimum altitude at that position. The controller immediately queried the pilot concerning his altitude, but received no response. According to the pilot, everything appeared normal until the aircraft collided with power lines and crashed. He stated that he believed he had been cleared to 2000 ft, then after intercepting the localizer, he 'began to descend to the glide slope.' An investigation revealed the aircraft had collided with power lines at an elevation of about 1525 ft. The accident occurred near the outer marker. The minimum altitude at the outer marker was 2409 ft. No preimpact system malfunction or failure was found.