Cessna 210 Centurion / Turbo Centurion 210C; 210D
Flight Crew Instrument
Flight Crew Last 90 Days 10; Flight Crew Total 750; Flight Crew Type 250
Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Requested ATC Assistance / Clarification; Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport
Aircraft
Departing ZZZ; navigating via GPS and Aspen PFD HSI (Primary Flight Display; Horizontal Situation Indicator).  Inadvertent off course route up to 45 degrees due to incorrect heading display on the Aspen PFD; the primary navigation source for the GPSS steering on the autopilot.  ATC informed of incorrect route.  Corrected course and rebooted Aspen; with no resolution.  Elected to return to departure airport for further troubleshooting on the ground.  ATC provided no-gyro turn assistance. Visual approach to ZZZ with uneventful landing. Aspen was indicating correctly on run up and pre-takeoff checks. Problem with the displayed heading on the Aspen PFD continued on the ground; although to a lesser degree; while taxiing.  During consultation with avionics shop; realized a pair of speakers being transported had been inadvertently placed in the passenger cabin below the roof mounted magnetometer for the Aspen.  Confirmed heading interference via Aspen configuration screens with speaker movement. After removal of the speakers (large magnets); confirmed proper Aspen indications on the ground.  Refiled IFR flight plan and departed on 3.5 hour flight with no further abnormalities.
C210 Pilot reported GPSS unreliability due to speaker magnet interference with magnetometer.